dt-bindings: add binding for USBSS-DRD controller.

This patch aim at documenting USB related dt-bindings for the
Cadence USBSS-DRD controller.

Signed-off-by: Pawel Laszczak <pawell@cadence.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>