ASoC: wm8974: add devicetree support

This adds devicetree support to the wm8974 codec driver.
With a DT-based kernel, there is no board-specific setting
to select the driver so allow it to be manually chosen.

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