ASoC: wm8974: configure pll and mclk divider automatically

This adds a set_sysclk() DAI op so the card driver can set the
input clock frequency.  If this is done, the pll and mclk divider
are configured to produce the required 256x fs clock when the
sample rate is set by hw_params().

These additions make the codec work with the simple-card driver.
Card drivers calling set_pll() and set_clkdiv() directly are
unaffected.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>