RAS: SGI: Add platform handler for RAS interrupts

Add a platform specific handler for RAS interrupts and configure the
platform RAS interrupts for EL3 handling. The interrupt handler passes
control to StandaloneMM code executing in S-EL0, which populates the
CPER buffer with relevant error information. The handler subsequently
invokes the SDEI client which processes the information in the error
information in the CPER buffer. The helper functions
plat_sgi_get_ras_ev_map and plat_sgi_get_ras_ev_map_size would be
defined for sgi platforms in the subsequent patch, which adds sgi575
specific RAS changes.

Change-Id: I490f16c15d9917ac40bdc0441659b92380108d63
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>