ASoC: mxs: add mxs-sgtl5000 machine driver

The driver only supports playback firstly.
For recording, as we have to use two saif instances to implement full
duplex (playback & recording) due to hardware limitation, we need to
figure out a good design to fit in ASoC.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Tested-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>