arm64: Update dram end address while randomizing base

Update the physical end address of available RAM in calculation
used for randomizing the memory base address base.

Change-Id: Id32260d5c15ec4363ab62755238a10cc4dfaabe8
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>