dt-bindings: usb: cdns,usb3: document USBSSP controller support

dt-bindings: usb: cdns,usb3: document USBSSP controller support

Update the Cadence USBSS DRD binding to document that it also covers
the USBSSP (SuperSpeed Plus, USB 3.1 gen2x1) controller. Both USBSS
and USBSSP share the same DRD/OTG register interface, so the driver
auto-detects the controller version at runtime: no additional
compatible string is needed.

Changes to the binding:
- Update title and add description
- maximum-speed: add super-speed-plus

Assisted-by: Cursor:claude-4.6-opus
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Peter Chen <peter.chen@cixtech.com>
Link: https://patch.msgid.link/20260421023459.506145-2-peter.chen@cixtech.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>