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>
diff --git a/docs/plat/hikey.rst b/docs/plat/hikey.rst
index 372d388..d7a01a8 100644
--- a/docs/plat/hikey.rst
+++ b/docs/plat/hikey.rst
@@ -78,7 +78,7 @@
        EDK2_OUTPUT_DIR=${EDK2_DIR}/Build/HiKey/${BUILD_OPTION}_${AARCH64_TOOLCHAIN}
        # Build fastboot for Trusted Firmware-A. It's used for recovery mode.
        cd ${BUILD_PATH}/atf-fastboot
-       CROSS_COMPILE=aarch64-linux-gnu- make PLAT=hikey DEBUG=1
+       CROSS_COMPILE=aarch64-none-elf- make PLAT=hikey DEBUG=1
        # Convert DEBUG/RELEASE to debug/release
        FASTBOOT_BUILD_OPTION=$(echo ${BUILD_OPTION} | tr '[A-Z]' '[a-z]')
        cd ${EDK2_DIR}