ARM platforms: Include BL2U's RO section in total memory region

ARM platforms: Include BL2U's RO section in total memory region

This patch changes the base address of the "total" Trusted SRAM region
seen by the BL2U image. It used to start just after BL2U's read-only
section (i.e. at address BL2U_RO_LIMIT), it now starts from the base
address of the BL2U image (i.e. at address BL2U_BASE). In other words,
the "total" memory region now includes BL2U's own read-only section.

This does not change BL2U's resulting memory mappings because the
read-only section was already mapped in BL2U, it just wasn't part of
this total memory region.

Change-Id: I2da16ac842469023b41904eaa8d13ed678d65671