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/rpi4.rst b/docs/plat/rpi4.rst
index 0f529c1..e87aef7 100644
--- a/docs/plat/rpi4.rst
+++ b/docs/plat/rpi4.rst
@@ -22,7 +22,7 @@
 
 .. code:: shell
 
-    CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi4 DEBUG=1
+    CROSS_COMPILE=aarch64-none-elf- make PLAT=rpi4 DEBUG=1
 
 Copy the generated build/rpi4/debug/bl31.bin to the SD card, either
 renaming it to ``armstub8.bin`` or adding an entry starting with ``armstub=``,