mm: add support for putting memory in a low-power state

The file /sys/devices/system/memory/low_power now exists.

Writing a physical address into this file will put this
section of memory into a low power state (retaining contents)
if the architecture and platform supports it.

Change-Id: I70592d37f1091a1b533f2374546ba67b50ea7d30
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>
(cherry picked from commit 1f4d1c8e295aaf66b23309caa0d03b09b7009b99)

Conflicts:

	drivers/base/memory.c
	include/linux/memory_hotplug.h