ASoC: fsl_sai: Add clock controls for SAI

The SAI mainly has the following clocks:
  bus clock
    control and configure registers and to generate synchronous
    interrupts and DMA requests.

  mclk1, mclk2, mclk3
    to generate the bit clock when the receiver or transmitter is
    configured for an internally generated bit clock.

So this patch adds these clocks and their clock controls to the driver.

[ To concern the old DTB cases, I've added a bit of extra code to make
  the driver compatible with them. And by marking clock NULL if failed
  to get, the clk_prepare() or clk_get_rate() would easily return 0
  so no further path should be broken. -- by Nicolin ]

Signed-off-by: Nicolin Chen <Guangyu.Chen@freescale.com>
Acked-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>