Add physical memory hotremove API

This provides the physical memory hotremove API (this is needed since our
physical memory removal is done by powering it off, which needs
to be requested by userspace, not by someone physically pulling memory
out of a machine).

Change-Id: Ic34426a91a1aac2bd4a45677ee00c2b7a3f84746
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>