ASoC: davinci-mcasp: Data source (bus) selection support

The audio data to/from McASP can be sent/received via two method:
Via the data port (preferred) or via the configuration bus.
Currently the driver assumes that all data communication will be done via
the data port.
This patch adds support for selecting the configuration port as data
interface.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>