Migrate to Linaro Release 16.12

Migrate to Linaro Release 16.12

This Linaro release updates both the binaries and the toolchain:
Linaro binaries upgraded 16.06 --> 16.12
AArch64 compiler upgraded 15.05 (gcc 4.9) --> 5.3-2015.05 (gcc 5.3)
AArch32 compiler upgraded 15.05 (gcc 4.9) --> 5.3-2015.05 (gcc 5.3)

The ARM TF codebase has been tested against these new binaries. This patch
updates the User Guide to reflect that the 16.12 release is now a supported
Linaro Release.

Change-Id: I6247e820f591df7d05df4f622ee45a3abf2c2d72
Signed-off-by: David Cunado <david.cunado@arm.com>