platforms/rd: fix incorrect definition of EDK2_ENABLE_RAS

EDK2_ENABLE_RAS compilation flag was incorrectly being defined for
builds with RAS disabled. Fix this for all the SGI/RD platfotms.

Change-Id: I01209f6f03ca26c3b2e16c415856630cbf45c6e2
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>