dt-binding: usb: add "super-speed-plus"

This commit adds "super-speed-plus" to valid argument list of
"maximum-speed" property.

Signed-off-by: JC Kuo <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/usb/generic.txt

···
 
 Optional properties:
  - maximum-speed: tells USB controllers we want to work up to a certain
-			speed. Valid arguments are "super-speed", "high-speed",
-			"full-speed" and "low-speed". In case this isn't passed
-			via DT, USB controllers should default to their maximum
-			HW capability.
+			speed. Valid arguments are "super-speed-plus",
+			"super-speed", "high-speed", "full-speed" and
+			"low-speed". In case this isn't passed via DT, USB
+			controllers should default to their maximum HW
+			capability.
  - dr_mode: tells Dual-Role USB controllers that we want to work on a
 			particular mode. Valid arguments are "host",
 			"peripheral" and "otg". In case this attribute isn't