OMAP: McBSP: Function to query the FIFO size

Users of McBSP can use the omap_mcbsp_get_fifo_size function to query
the size of the McBSP FIFO.
The function will return the FIFO size in words (the HW maximum).

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolsfonmicro.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>