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 <[e-mail]>
Reviewed-by: Heiko Stuebner <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

Documentation/devicetree/bindings/phy/phy-rockchip-typec.txt

···
  - resets : a list of phandle + reset specifier pairs
  - reset-names : string reset name, must be:
 		 "uphy", "uphy-pipe", "uphy-tcphy"
+
+Optional properties:
  - extcon : extcon specifier for the Power Delivery
 
 Required nodes : a sub-node is required for each port the phy provides.