docs: GCC toolchain upgrade to version 9.2-2019.12

This toolchain provides multiple cross compilers and are publicly
available on developer.arm.com

We build TF-A in CI using:
AArch32 bare-metal target (arm-none-eabi)
AArch64 ELF bare-metal target (aarch64-none-elf)

Change-Id: I910200174d5bad985504d1af4a1ae5819b524003
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/docs/plat/arm/fvp/index.rst b/docs/plat/arm/fvp/index.rst
index b6396b9..37010e1 100644
--- a/docs/plat/arm/fvp/index.rst
+++ b/docs/plat/arm/fvp/index.rst
@@ -223,7 +223,7 @@
 
 .. code:: shell
 
-    CROSS_COMPILE=aarch64-linux-gnu-  \
+    CROSS_COMPILE=aarch64-none-elf-  \
     make PLAT=fvp DEBUG=1             \
     RESET_TO_BL31=1                   \
     ARM_LINUX_KERNEL_AS_BL33=1        \