DMM: Fix for movable bytes near end of address space

To prevent overflow near 4GB memory address, the rounding down of
memory addresses needs to be propagated to the memory hotplug logic.
Checking if a given pfn is part of physical ram allows us to do this.
Also while walking through system ram, we need to take care of
overflow at high memory address.

Change-Id: Id962cf93906888783a807fe89f2be4ba91b2c5d6
Signed-off-by: Hanumant Singh <hanumant@codeaurora.org>