ASoC: ak4642: Add pll select support

Current ak4642 was not able to select pll.
This patch add support it.
It still expect PLL base input pin is MCKI.
see Table 5 "setting of PLL Mode" of datasheet

Signed-off-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>