Documentation: rockchip-dw-mshc: add description for rk3228

Add "rockchip,rk3228-dw-mshc", "rockchip,rk3288-dw-mshc" for
dwmmc on rk322x platform.
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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

@@ -15,6 +15,7 @@ Required Properties:
 	- "rockchip,rk3288-dw-mshc": for Rockchip RK3288
 	- "rockchip,rv1108-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RV1108
 	- "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
 	- "rockchip,rk3368-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RK3368
 	- "rockchip,rk3399-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RK3399