pmem: add support for reusable pmem

Individual pmem devices can now be marked as reusable. The
memory will be reused through fmem.

Change-Id: Id71c2e0c42f093420c241771d7a2620ed2accfc4
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Include: linux/fmem.h

Change-Id: Iea427b3ffd2b97808d0b71ef0f53c89401adda8d