dt-bindings: rockchip-dw-mshc: add description for rk3399

Add "rockchip,rk3399-dw-mshc", "rockchip,rk3288-dw-mshc" for
dwmmc on rk3399 platform.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

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

@@ -15,6 +15,7 @@ Required Properties:
 	- "rockchip,rk3288-dw-mshc": for Rockchip RK3288
 	- "rockchip,rk3036-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RK3036
 	- "rockchip,rk3368-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RK3368
+	- "rockchip,rk3399-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RK3399
 
 Optional Properties:
 * clocks: from common clock binding: if ciu_drive and ciu_sample are