Add devicetree bindings for i2s controller found on rk3399
processors from rockchip.

It's helpful to add full set of compatible strings for serials
of Rockchip SoCs (rk3066, rk3188, rk3288, rk3399).
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: NMark Brown <broonie@kernel.org>