ASoC: Beagle: Add support for 4 channel

This patch adds support for the four channel TDM mode
on Beagle board.

Depending on the channel count, the interface needs to be
configured differently (I2S for stereo DSP_A for four channels)

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>