ASoC: rt5640: add device tree support

Modify the RT5640 driver to parse platform data from device tree. Write
a DT binding document to describe those properties.

Slight re-ordering of rt5640_i2c_probe() to better fit the DT parsing.

Since ldo1_en is optional, guard usage of it with gpio_is_valid(), rather
than open-coding an if (gpio) check.

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