dt-bindings: phy-rockchip-typec: move extcon property to be optional.

The extcon property is used to detect the cable-state but some boards
just connect the type-c phy to a regular USB-A connector without any
power-delivery and thus no controller reporting the cable-state.
So the extcon property is not really a required property, move it to be
optional instead.

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>