ARM: McBSP: Add support for omap4 in McBSP driver

McBSP module in OMAP4 needs to be able to set its tx/rx threshold
and enable the transmitter/receiver when starting an audio stream.

Signed-off-by: Jorge Eduardo Candelaria <jorge.candelaria@ti.com>
Signed-off-by: Margarita Olaya Cabrera <magi.olaya@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>