ASoC: wm8962: Add device tree binding

Document the device tree binding for the WM8962 codec, and modify the
driver to extract platform data from the device tree, if present.

Based on work of WM8903 by Stephen Warren <swarren@nvidia.com>

Signed-off-by: Nicolin Chen <b42378@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>