ASoC: wm8978: Add OF device ID table

The driver doesn't have a struct of_device_id table but supported devices
are registered via Device Trees. This is working on the assumption that a
I2C device registered via OF will always match a legacy I2C device ID and
that the MODALIAS reported will always be of the form i2c:<device>.

But this could change in the future so the correct approach is to have an
OF device ID table if the devices are registered via OF.

Before this patch:

$ modinfo sound/soc/codecs/snd-soc-wm8978.ko | grep alias
alias:          i2c:wm8978

After this patch:

$ modinfo sound/soc/codecs/snd-soc-wm8978.ko | grep alias
alias:          i2c:wm8978
alias:          of:N*T*Cwlf,wm8978C*
alias:          of:N*T*Cwlf,wm8978

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>