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

Commit 98898f3b ("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: 98898f3b ("phy: rockchip-inno-usb2: support otg-port for rk3399")
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>