ASoC: rockchip: add bindings for i2s

This patch add bindings for rk3036/rk3228/rk3328/rk3366 i2s.

Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>