RealView: Remove duplicated #define REALVIEW_SYS_FLAGS* statements

The platsmp.c file defines the REALVIEW_SYS_FLAGS* macros which are
already present in platform.h. Just use the latter.

Signed-off-by: Colin Tuckley <colin.tuckley@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>