configs/sgi575: allow support for RAS to be optional

In order to support build of SGI-575 platform with and without
RAS support, add new build defintions ARM_TF_ENABLE_SPM and
ARM_TF_RAS_EXTENSION. These have to be set to '1' in order to
support SPM and RAS support.

Change-Id: I2556919bcfe8bf6f3503cb8ed619e902c2fa9d73
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>