Rule 5.1: External identifiers shall be distinct

Some of the identifier names in the GICv3 driver were so long that the
first 31 characters were identical. This patch shortens these names to
make sure they are different.

Fixed for:
    LOG_LEVEL=50 PLAT=fvp

Change-Id: Iecd551e3a015d144716b87b42c83dd3ab8c34d90
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>