ASoC: arizona: Move selection of FLL REFCLK into init

In preparation for additional features on the FLL this patch moves the
code selecting the REFCLK source based on the 32kHz clock into the FLL
initialisation function.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>