ASoC: fsl_sai: Add asynchronous mode support

SAI supports these operation modes:
1) asynchronous mode
   Both Tx and Rx are set to be asynchronous.
2) synchronous mode (Rx sync with Tx)
   Tx is set to be asynchronous, Rx is set to be synchronous.
3) synchronous mode (Tx sync with Rx)
   Rx is set to be asynchronous, Tx is set to be synchronous.
4) synchronous mode (Tx/Rx sync with another SAI's Tx)
5) synchronous mode (Tx/Rx sync with another SAI's Rx)

* 4) and 5) are beyond this patch because they are related with another SAI.

As the initial version of this SAI driver, it supported 2) as default while
the others were totally missing.

So this patch just adds supports for 1) and 3).

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>