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/juno/index.rst b/docs/plat/arm/juno/index.rst
index 6429ede..cf328fa 100644
--- a/docs/plat/arm/juno/index.rst
+++ b/docs/plat/arm/juno/index.rst
@@ -136,7 +136,7 @@
 
       .. code:: shell
 
-          export CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-linux-gnu-
+          export CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-none-elf-
 
    -  The following parameters should be used to build BL1 and BL2 in AArch64
       and point to the BL32 file.