mmc: omap_hsmmc: add runtime pm support

* Add runtime pm support to HSMMC host controller.
* Use runtime pm API to enable/disable HSMMC clock.
* Use runtime autosuspend APIs to enable auto suspend delay.

Based on OMAP HSMMC runtime implementation by Kevin Hilman and
Kishore Kadiyala.

Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>