ASoC: Add WM8912 DAC support

The WM8912 is a DAC only device register compatible with the WM8904
CODEC with ADC portions omitted.  Support it within the WM8904 driver
based on the configured I2C device name.

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