Limit BL3-1 read/write access to SRAM

At present BL3-1 has access to all of the SRAM, including
regions that are mapped as read-only and non-cacheable by other
firmware images.

This patch restricts BL3-1 to only be able to read/write from
memory used for its own data sections

Change-Id: I26cda1b9ba803d91a9eacda768f3ce7032c6db94