OMAP: McBSP: Add pm runtime support

Add pm runtime support for McBSP driver.
Reference to fclk is not removed because it is required when the
functional clock is switched from one source to another.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>