ASoC: dt-bindings: add rockchip i2s bindings

Add devicetree bindings for i2s controller found on
rk3066, rk3188 and rk3288 processors from rockchip.

Signed-off-by: Jianqun Xu <xjq@rock-chips.com>
Signed-off-by: Mark Brown <broonie@linaro.org>