ASoC: TWL4030: Add digital loopback support

This patch adds the digital loopback/bypass support for twl4030 codec.

The digital loopback will let the digimic0 (routed in the TX1 capture path
inside of TWL4030) data to be routed back to the RX2 playback path
(I2S stereo). It can also route the analog capture date routed through the
TX1 back to RX2.

Effectively the digital loopback is routing the audio from the TX1 capture path
to the RX2 playback path.

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