dt-bindings: mmc: rockchip-dw-mshc: add description for px30

Add "rockchip,px30-dw-mshc", "rockchip,rk3288-dw-mshc" for
dwmmc on px30 platform.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Liang Chen <cl@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.txt

@@ -14,6 +14,7 @@ Required Properties:
 							before RK3288
 	- "rockchip,rk3288-dw-mshc": for Rockchip RK3288
 	- "rockchip,rv1108-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RV1108
+	- "rockchip,px30-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip PX30
 	- "rockchip,rk3036-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RK3036
 	- "rockchip,rk3228-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RK322x
 	- "rockchip,rk3328-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RK3328