plat/arm/sgi: add RMM dram region MMU mapping to BL2

As a preparation to support BL1->BL2 based boot flow, add RMM DRAM
region to BL2 MMU mapping to allow BL2 to load the RMM image.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>