OMAP: McBSP: Add link DMA mode selection

It adds a new sysfs file, where the user can configure the mcbsp mode to use.
If the mcbsp channel is in use, it does not allow the change.
Than in omap_pcm_open we can call the omap_mcbsp_get_opmode to get the mode,
store it, than use it to implement the different modes.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@nokia.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>