[TF-A] GCC toolchain upgrade in CI

Hi, We have upgraded Trusted Firmware-A CI to use the GCC toolchain version 9.2.-2019.12 publicly available through developer.arm.com [1]. The relevant documentation has been updated [2] to reflect the use of these cross compilers. We request the platform maintainers to update their respective platform documentation ( trusted-firmware-a/docs/plat/*.rst) as needed.

AArch32 bare-metal target (arm-none-eabi) AArch64 ELF bare-metal target (aarch64-none-elf)

[1] https://developer.arm.com/tools-and-software/open-source-software/developer-...

[2] https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=f35e5...

Thanks, Madhukar