rpi4: Fix documentation of armstub config entry ...

It's in fact mandatory. Seen with RPi firmware 1.20190925.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Change-Id: I80739b74f25817294adc52cfd26a3ec59c06f892

Revert "GCC: Upgrade to version 9.2-2019.12 of toolchain" ...

This reverts commit de9bf1d.

Change-Id: Iebb6297ce290a10ee850bf6a9c71e7eb530b085f

GCC: Upgrade to version 9.2-2019.12 of toolchain ...

This toolchain provides multiple cross compilers and are publicly
available on www.developer.arm.com

We thoroughly test TF-A in CI using:
AArch32 bare-metal target (arm-none-eabi)
AArch64 ELF bare-metal target (aarch64-none-elf)

Change-Id: I2360a3ac6705c68dca781b85e9894867df255b3e
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>

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>