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>