dt-bindings: usb: mtu3: remove optional pinctrls

Remove optional pinctrls due to using FORCE/RG_IDDIG to implement
manual switch function.
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/devicetree/bindings/usb/mediatek,mtu3.txt

@@ -30,9 +30,10 @@ Optional properties:
 	when supports dual-role mode.
  - vbus-supply : reference to the VBUS regulator, needed when supports
 	dual-role mode.
- - pinctl-names : a pinctrl state named "default" must be defined,
-	"id_float" and "id_ground" are optinal which depends on
-	"mediatek,enable-manual-drd"
+ - pinctrl-names : a pinctrl state named "default" is optional, and need be
+	defined if auto drd switch is enabled, that means the property dr_mode
+	is set as "otg", and meanwhile the property "mediatek,enable-manual-drd"
+	is not set.
  - pinctrl-0 : pin control group
 	See: Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt