rdfremont: use SRAM area for MHUv3 communication

SRAM reserved area ranging from 0x80000 to 0x100000 is used for MHU
outband message buffer.
This patch adds the necessary region mapping and gpt entries for using
SRAM area as shared buffer for MHUv3 communication.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Signed-off-by: Sayanta Pattanayak <sayanta.pattanayak@arm.com>
Change-Id: I215cb2d7e21acada61e993b28c12c43332122ca5