ASoC: rt5640: add an of_match table

ASoC: rt5640: add an of_match table

Add a device tree match table. This serves to make the driver's support
of device tree more explicit. Perhaps the fallback for DT matching to
using the i2c_device_id table will go away one day, since it fails in
face of devices from different vendors with the same name.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>