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/allwinner.rst b/docs/plat/allwinner.rst
index a1e0659..5c905d8 100644
--- a/docs/plat/allwinner.rst
+++ b/docs/plat/allwinner.rst
@@ -26,13 +26,13 @@
 
 .. code:: shell
 
-    make CROSS_COMPILE=aarch64-linux-gnu- PLAT=sun50i_a64 DEBUG=1 bl31
+    make CROSS_COMPILE=aarch64-none-elf- PLAT=sun50i_a64 DEBUG=1 bl31
 
 To build for machines with an H6 SoC:
 
 .. code:: shell
 
-    make CROSS_COMPILE=aarch64-linux-gnu- PLAT=sun50i_h6 DEBUG=1 bl31
+    make CROSS_COMPILE=aarch64-none-elf- PLAT=sun50i_h6 DEBUG=1 bl31
 
 .. _U-Boot documentation: http://git.denx.de/?p=u-boot.git;f=board/sunxi/README.sunxi64;hb=HEAD