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/rockchip.rst b/docs/plat/rockchip.rst
index b7c43fb..2c3c38f 100644
--- a/docs/plat/rockchip.rst
+++ b/docs/plat/rockchip.rst
@@ -35,7 +35,7 @@
 
 For AARCH64 architectures the build command looks like
 
-    make CROSS_COMPILE=aarch64-linux-gnu- PLAT=rk3399 bl32
+    make CROSS_COMPILE=aarch64-none-elf- PLAT=rk3399 bl32
 
 while AARCH32 needs a slightly different command