dt-bindings: add documentation for rk1108 pinctrl

This adds the dt-binding documentation for rk1108 pinctrl
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/rockchip,pinctrl.txt

@@ -19,10 +19,11 @@ The pins are grouped into up to 5 individual pin banks which need to be
 defined as gpio sub-nodes of the pinmux controller.
 
 Required properties for iomux controller:
-  - compatible: one of "rockchip,rk2928-pinctrl", "rockchip,rk3066a-pinctrl"
-		       "rockchip,rk3066b-pinctrl", "rockchip,rk3188-pinctrl"
-		       "rockchip,rk3228-pinctrl", "rockchip,rk3288-pinctrl"
-		       "rockchip,rk3368-pinctrl", "rockchip,rk3399-pinctrl"
+  - compatible: one of "rockchip,rk1108-pinctrl", "rockchip,rk2928-pinctrl"
+		       "rockchip,rk3066a-pinctrl", "rockchip,rk3066b-pinctrl"
+		       "rockchip,rk3188-pinctrl", "rockchip,rk3228-pinctrl"
+		       "rockchip,rk3288-pinctrl", "rockchip,rk3368-pinctrl"
+		       "rockchip,rk3399-pinctrl"
   - rockchip,grf: phandle referencing a syscon providing the
 	 "general register files"