ASoC: WM8903: Add device tree binding

Document the device tree binding for the WM8903 codec, and modify the
driver to extract platform data from the device tree, if present.

Based on work by John Bonesio, but significantly reworked since then.

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