mfd: Add omap-usbhs runtime PM support

The usbhs core driver does not enable/disable the interface and
functional clocks; These clocks are handled by hwmod and runtime pm,
hence insted of the clock enable/disable, the runtime pm APIS are
used. however,the port clocks and tll clocks are handled
by the usbhs core.

Signed-off-by: Keshava Munegowda <keshava_mgowda@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>