OMAP: McBSP: Add link DMA mode selection

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 <[email protected]>
Signed-off-by: Eduardo Valentin <[email protected]>
Acked-by: Tony Lindgren <[email protected]>
Signed-off-by: Mark Brown <[email protected]>