dt-binding: usb: Include USB SSP rates in GenXxY

According to the USB 3.2 spec, a SuperSpeed Plus device can operate at
gen2x2, gen2x1, or gen1x2. If the USB controller device supports
multiple lanes at different transfer rates, the user can specify the HW
capability via these new speed strings:

"super-speed-plus-gen2x2"
"super-speed-plus-gen2x1"
"super-speed-plus-gen1x2"

If the argument is simply "super-speed-plus", USB controllers should
default to their maximum transfer rate and number of lanes.
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NThinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/cc7cc15f87e209c9963f19129f51398cdc374358.1611106162.git.Thinh.Nguyen@synopsys.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/usb.yaml

@@ -54,6 +54,9 @@ properties:
       - high-speed
       - super-speed
       - super-speed-plus
+      - super-speed-plus-gen2x1
+      - super-speed-plus-gen1x2
+      - super-speed-plus-gen2x2
 
 additionalProperties: true