mm/hmm/devmem: dummy HMM device for ZONE_DEVICE memory

mm/hmm/devmem: dummy HMM device for ZONE_DEVICE memory

This introduce a dummy HMM device class so device driver can use it to
create hmm_device for the sole purpose of registering device memory.  It
is useful to device driver that want to manage multiple physical device
memory under same struct device umbrella.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jérôme Glisse <[email protected]>
Signed-off-by: Evgeny Baskakov <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
Signed-off-by: Mark Hairgrove <[email protected]>
Signed-off-by: Sherry Cheung <[email protected]>
Signed-off-by: Subhash Gutti <[email protected]>
Cc: Aneesh Kumar <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: David Nellans <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Ross Zwisler <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Bob Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>