Move frequently defined constants to config.h

This commit aims to reduce duplication by moving
constants that are commonly defined across the
code-base into a conventient 'config.h' header,
that can be included wherever needed.

Change-Id: Ibcf6979c37e0e664c9d9604116fa41d7619e5e43
Signed-off-by: Luke Hutton <luke.hutton@arm.com>