libata: Expose TRIM capability in sysfs

Create a sysfs "trim" attribute for each ata_device that displays
whether DSM TRIM is "unsupported", "unqueued", "forced_unqueued"
(blacklisted) or "queued".

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Tejun Heo <tj@kernel.org>