RAS: SGI575: Add platform specific RAS changes

Add platform specific changes needed to add support for the RAS
feature on SGI575 platform, including adding a mapping for the
CPER buffer being used on SGI575 platform.

Change-Id: I01a982e283609b5c48661307906346fa2738a43b
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>