ASoC: mxs-saif: set a base clock rate for EXTMASTER mode work

Set an initial clock rate for the saif internal logic to work
properly. This is important when working in EXTMASTER mode that
uses the other saif's BITCLK&LRCLK but it still needs a basic
clock which should be fast enough for the internal logic.

Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>