Add a sysfs entry to turn on priority information being passed
to a ATA device. By default this feature is turned off.

This patch depends on ata: Enabling ATA Command Priorities

tj: Renamed ncq_prio_on to ncq_prio_enable and removed trivial
    ata_ncq_prio_on() and open-coded the test.
Signed-off-by: NAdam Manzanares <adam.manzanares@hgst.com>
Signed-off-by: NTejun Heo <tj@kernel.org>