arm64: mm/memblock: Update memory limit calculation

The system RAM region would not be ideally contiguous from
start to end of DRAM. Detect any offset/holes present in the
DDR region and update the memory limit being set, so that the
DRAM end address is aligned to the memory block size. This is
a requirement for mem-offline framework to work properly.

Change-Id: I97cb0cce70e7c414ae2d200a4a3f714d145c0e64
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>