configs/sgi575: enable SPM and RAS by default in tf-a build

Enable SPM and RAS build flags by default for TF-A build. StandaloneMM
is assumed to be always used as secure partition on SGI platforms and
so these flags are being enabled by default.

Change-Id: I33e993ab3b3d85a9fc633464a908709ed4a80b69
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>