ASoC: ad1938: let soc-core dapm handle PLL power

PM architecture of ad1938 is simple, we don't need a bundle of functions like
ad1938_pll_powerctrl, ad1938_set_bias_level for only PLL. A dapm supply will
handle on/off of PLL.
Since soc-core can poweron/off PLL on-demand, we don't need to poweron/off PLL
in suspend/resume entries too.

Signed-off-by: Barry Song <21cnbao@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>