rpi4: Add initial documentation file

As the Raspberry Pi4 port is now in a usable state, add the build
instructions together with some background information to the
documentation directory.
The port differs quite a bit from the Raspberry Pi 3, so we use a
separate file for that.

Change-Id: I7d9f5967fdf3ec3bfe97d78141f59cbcf03388d4
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

docs/plat/rpi4.rst

+Raspberry Pi 4
+==============
+
+The `Raspberry Pi 4`_ is an inexpensive single-board computer that contains four
+Arm Cortex-A72 cores. Also in contrast to previous Raspberry Pi versions this
+model has a GICv2 interrupt controller.
+
+This port is a minimal port to support loading non-secure EL2 payloads such
+as a 64-bit Linux kernel. Other payloads such as U-Boot or EDK-II should work
+as well, but have not been tested at this point.
+
+**IMPORTANT NOTE**: This port isn't secure. All of the memory used is DRAM,
+which is available from both the Non-secure and Secure worlds. The SoC does
+not seem to feature a secure memory controller of any kind, so portions of
+DRAM can't be protected properly from the Non-secure world.
+
+Build Instructions
+------------------
+
+There are no real configuration options at this point, so there is only
+one universal binary (bl31.bin), which can be built with:
+
+.. code:: shell
+
+    CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi4 DEBUG=1
+
+Copy the generated build/rpi4/debug/bl31.bin to the SD card, either
+renaming it to ``armstub8.bin`` or adding an entry starting with ``armstub=``,
+then followed by the respective file name to ``config.txt``.
+You should have AArch64 code in the file loaded as the "kernel", as BL31
+will drop into AArch64/EL2 to the respective load address.
+arm64 Linux kernels are known to work this way.
+
+Other options that should be set in ``config.txt`` to properly boot 64-bit
+kernels are:
+
+::
+
+    enable_uart=1
+    arm_64bit=1
+    enable_gic=1
+
+The BL31 code will patch the provided device tree blob in memory to advertise
+PSCI support, also will add a reserved-memory node to the DT to tell the
+non-secure payload to not touch the resident TF-A code.
+
+If you connect a serial cable between the Mini UART and your computer, and
+connect to it (for example, with ``screen /dev/ttyUSB0 115200``) you should
+see some text from BL31, followed by the output of the EL2 payload.
+The command line provided is read from the ``cmdline.txt`` file on the SD card.
+
+TF-A port design
+----------------
+
+In contrast to the existing Raspberry Pi 3 port this one here is a BL31-only
+port, also it deviates quite a lot from the RPi3 port in many other ways.
+There is not so much difference between the two models, so eventually those
+two could be (more) unified in the future.
+
+As with the previous models, the GPU and its firmware are the first entity to
+run after the SoC gets its power. The on-chip Boot ROM loads the next stage
+(bootcode.bin) from flash (EEPROM), which is again GPU code.
+This part knows how to access the MMC controller and how to parse a FAT
+filesystem, so it will load further compononents and configuration files
+from the first FAT partition on the SD card.
+
+To accommodate this existing way of configuring and setting up the board,
+we use as much of this workflow as possible.
+If bootcode.bin finds a file called ``armstub8.bin`` on the SD card or it gets
+pointed to such code by finding a ``armstub=`` key in ``config.txt``, it will
+load this file to the beginning of DRAM (address 0) and execute it in
+AArch64 EL3.
+But before doing that, it will also load a "kernel" and the device tree into
+memory. The load addresses have a default, but can also be changed by
+setting them in ``config.txt``. If the GPU firmware finds a magic value in the
+armstub image file, it will put those two load addresses in memory locations
+near the beginning of memory, where TF-A code picks them up.
+
+To keep things simple, we will just use the kernel load address as the BL33
+entry point, also put the DTB address in the x0 register, as requested by
+the arm64 Linux kernel boot protocol. This does not necessarily mean that
+the EL2 payload needs to be a Linux kernel, a bootloader or any other kernel
+would work as well, as long as it can cope with having the DT address in
+register x0. If the payload has other means of finding the device tree, it
+could ignore this address as well.