dt-bindings: phy-rockchip-inno-usb2: add documentation for extcon property

Commit 98898f3bc83c8 ("phy: rockchip-inno-usb2: support otg-port for
rk3399") introduces the extcon property that is used to detect the
cable-state. Document this property in the documentation binding.

Fixes: 98898f3bc83c8 ("phy: rockchip-inno-usb2: support otg-port for rk3399")
Signed-off-by: Enric Balletbo i Serra <[e-mail]>
Reviewed-by: Heiko Stuebner <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

Documentation/devicetree/bindings/phy/phy-rockchip-inno-usb2.txt

···
 		 register files". When set driver will request its
 		 phandle as one companion-grf for some special SoCs
 		 (e.g RV1108).
+ - extcon : phandle to the extcon device providing the cable state for
+		 the otg phy.
 
 Required nodes : a sub-node is required for each port the phy provides.
 		 The sub-node name is used to identify host or otg port,