docs: GCC toolchain upgrade to version 9.2-2019.12

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

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

Change-Id: I910200174d5bad985504d1af4a1ae5819b524003
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
4 files changed