rpi3: Fix kernel boot documentation

The order of the arguments of memmap was swapped. The old command was
reserving 256 MiB from the 16 MiB barrier, it should be reserving only
16 MiB at the 256 MiB barrier.

It worked because the memory used by the Trusted Firmware was reserved
anyway.

Change-Id: I3fefcfc0105ecf05ba5606517bc3236f4eb24ceb
Tested-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

docs/plat/rpi3.rst

@@ -134,7 +134,7 @@ secure platform!
     0x40000000 +-----------------+
 
 The area between **0x10000000** and **0x11000000** has to be protected so that
-the kernel doesn't use it. That is done by adding ``memmap=256M$16M`` to the
+the kernel doesn't use it. That is done by adding ``memmap=16M$256M`` to the
 command line passed to the kernel. See the `Setup SD card`_ instructions to see
 how to do it.
 
@@ -317,7 +317,7 @@ untouched). They have been tested with the image available in 2018-03-13.
    bootloader will look for a file called ``armstub8.bin`` and load it at
    address **0x0** instead of a predefined one.
 
-4. Open ``cmdline.txt`` and add ``memmap=256M$16M`` to prevent the kernel from
+4. Open ``cmdline.txt`` and add ``memmap=16M$256M`` to prevent the kernel from
    using the memory needed by TF-A. If you want to enable the serial port
    "Mini UART", make sure that this file also contains
    ``console=serial0,115200 console=tty1``.