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/poplar.rst b/docs/plat/poplar.rst
index 215f551..040b593 100644
--- a/docs/plat/poplar.rst
+++ b/docs/plat/poplar.rst
@@ -68,7 +68,7 @@
 
 .. code:: bash
 
-       make CROSS_COMPILE=aarch64-linux-gnu-  all fip SPD=none PLAT=poplar
+       make CROSS_COMPILE=aarch64-none-elf- all fip SPD=none PLAT=poplar
        BL33=u-boot.bin
 
 -  Build l-loader (generated the final fastboot.bin)