dt-bindings: net: dsa: b53: Document brcm,gpio-ctrl property

Add description for bcm63xx gpio-ctrl phandle which allows
access to registers that control phy functionality.

Signed-off-by: Kyle Hendry <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/dsa/brcm,b53.yaml

···
               - brcm,bcm63268-switch
           - const: brcm,bcm63xx-switch
 
+  brcm,gpio-ctrl:
+    description:
+      A phandle to the syscon node of the bcm63xx gpio controller
+      which contains phy control registers
+    $ref: /schemas/types.yaml#/definitions/phandle
+
 required:
   - compatible
   - reg