dmaengine: img-mdc: Add runtime PM

Add runtime PM support to disable the clock when the h/w is not in use.
The existing clock_prepare_enable is removed from probe() as the clock
is no longer permanently enabled.

Signed-off-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>