stm32mp1: Add platform documentation

Signed-off-by: Yann Gautier <yann.gautier@st.com>

docs/plat/stm32mp1.rst

+Trusted Firmware-A for STM32MP1
+===============================
+
+STM32MP1 is a microprocessor designed by STMicroelectronics
+based on a dual Arm Cortex-A7.
+It is an Armv7-A platform, using dedicated code from TF-A.
+
+
+Design
+------
+The STM32MP1 resets in the ROM code of the Cortex-A7.
+The primary boot core (core 0) executes the boot sequence while
+secondary boot core (core 1) is kept in a holding pen loop.
+The ROM code boot sequence loads the TF-A binary image from boot device
+to embedded SRAM.
+
+The TF-A image must be properly formatted with a STM32 header structure
+for ROM code is able to load this image.
+Tool stm32image can be used to prepend this header to the generated TF-A binary.
+
+At compilation step, BL2, BL32 and DTB file are linked together in a single
+binary. The stm32image tool is also generated and the header is added to TF-A
+binary. This binary file with header is named tf-a-stm32mp157c-ev1.stm32.
+It can then be copied in the first partition of the boot device.
+
+
+Memory mapping
+~~~~~~~~~~~~~~
+
+::
+
+    0x00000000 +-----------------+
+               |                 |   ROM
+    0x00020000 +-----------------+
+               |                 |
+               |       ...       |
+               |                 |
+    0x2FFC0000 +-----------------+ \
+               |                 | |
+               |       ...       | |
+               |                 | |
+    0x2FFD8000 +-----------------+ |
+               |    TF-A DTB     | | Embedded SRAM
+    0x2FFDC000 +-----------------+ |
+               |       BL2       | |
+    0x2FFEF000 +-----------------+ |
+               |       BL32      | |
+    0x30000000 +-----------------+ /
+               |                 |
+               |       ...       |
+               |                 |
+    0x40000000 +-----------------+
+               |                 |
+               |                 |   Devices
+               |                 |
+    0xC0000000 +-----------------+ \
+               |                 | |
+    0xC0100000 +-----------------+ |
+               |       BL33      | | Non-secure RAM (DDR)
+               |       ...       | |
+               |                 | |
+    0xFFFFFFFF +-----------------+ /
+
+
+Boot sequence
+~~~~~~~~~~~~~
+
+ROM code -> BL2 (compiled with BL2_AT_EL3) -> BL32 (SP_min) -> BL33 (U-Boot)
+
+
+Build Instructions
+------------------
+
+To build:
+
+.. code:: bash
+
+    make CROSS_COMPILE=arm-linux-gnueabihf- PLAT=stm32mp1 ARCH=aarch32 ARM_ARCH_MAJOR=7 AARCH32_SP=sp_min
+
+The following build options are supported:
+
+- ``ENABLE_STACK_PROTECTOR``: To enable the stack protection.