docs: Upgrade GCC toolchain to 9.2-2019.12

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

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

Change-Id: I9beeb45a8215f8be206b665a8b2cee7c692170e8
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>