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>

doc: Add more missing platforms ...

Add meson-g12a, qemu-sbsa and rpi4 to the documentation index so
that they will have their docs rendered and integrated into the
table of contents.

Change-Id: Id972bf2fee67312dd7bff29f92bea67842e62431
Signed-off-by: Paul Beesley <paul.beesley@arm.com>