dt-bindings: rockchip-dw-mshc: add RK3368 dw-mshc description

rk3368 dtsi file add dw-mshc compatible "rockchip,rk3368-dw-mshc"
but didn't add it into rockchip-dw-mshc.txt.
Signed-off-by: Shawn Lin <shawn.lin@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

@@ -13,6 +13,7 @@ Required Properties:
 	- "rockchip,rk2928-dw-mshc": for Rockchip RK2928 and following,
 							before RK3288
 	- "rockchip,rk3288-dw-mshc": for Rockchip RK3288
+	- "rockchip,rk3368-dw-mshc", "rockchip,rk3288-dw-mshc": for Rockchip RK3368
 
 Optional Properties:
 * clocks: from common clock binding: if ciu_drive and ciu_sample are