dt-bindings: pinctrl: ocelot: Add Luton SoC support

Add the documentation for the Microsemi Luton pinmuxing and gpio
controller.
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lore.kernel.org/r/20201106093118.965152-2-gregory.clement@bootlin.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/mscc,ocelot-pinctrl.txt

@@ -3,7 +3,8 @@ Microsemi Ocelot pin controller Device Tree Bindings
 
 Required properties:
  - compatible		: Should be "mscc,ocelot-pinctrl",
-			  "mscc,jaguar2-pinctrl" or "microchip,sparx5-pinctrl"
+			  "mscc,jaguar2-pinctrl", "microchip,sparx5-pinctrl"
+			  or "mscc,luton-pinctrl"
  - reg			: Address and length of the register set for the device
  - gpio-controller	: Indicates this device is a GPIO controller
  - #gpio-cells		: Must be 2.