dt-bindings: net: dsa: add MT7530 GPIO controller binding

Add device tree binding to support MT7530 GPIO controller.
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/dsa/mt7530.txt

@@ -76,6 +76,12 @@ phy-mode must be set, see also example 2 below!
  * mt7621: phy-mode = "rgmii-txid";
  * mt7623: phy-mode = "rgmii";
 
+Optional properties:
+
+- gpio-controller: Boolean; if defined, MT7530's LED controller will run on
+	GPIO mode.
+- #gpio-cells: Must be 2 if gpio-controller is defined.
+
 See Documentation/devicetree/bindings/net/dsa/dsa.txt for a list of additional
 required, optional properties and how the integrated switch subnodes must
 be specified.