board/rdfremont: Add and map available shared SRAM

Update PLAT_ARM_TRUSTED_SRAM and PLAT_ARM_RESERVED_SRAM regions
which represents SRAM on the platform shared among AP,RSS,SCP and
MCP. Map the SRAM region as Root PAS in RME enabled system. Fix
the problem of visibility of its updated value to other subsytems.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Change-Id: I2a357f802a9702c28e586b25c55545bd16be2936