mm: sort out the DEVICE_PRIVATE Kconfig mess

The ZONE_DEVICE support doesn't depend on anything HMM related, just on
various bits of arch support as indicated by the architecture.  Also
don't select the option from nouveau as it isn't present in many setups,
and depend on it instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>