Resolve build errors flagged by GCC 6.2

With GCC 6.2 compiler, more C undefined behaviour is being flagged as
warnings, which result in build errors in ARM TF build.

This patch addresses issue caused by enums with values that exceed
maximum value for an int. For these cases the enum is converted to
a set of defines.

Change-Id: I5114164be10d86d5beef3ea1ed9be5863855144d
Signed-off-by: David Cunado <david.cunado@arm.com>