drm/omap: tpd12s015: gpio descriptor API

Migrated the gpio APIs to descriptor-interface based.

Signed-off-by: Manisha Agrawal <manisha.agrawal@ti.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>