dt-bindings: usb: DT bindings documentation for Broadcom IPROC USB Device controller.

The device node is used for UDCs integrated into Broadcom's
iProc family of SoCs'. The UDC is based on Synopsys Designware
Cores AHB Subsystem USB Device Controller IP.

Signed-off-by: Raviteja Garimella <raviteja.garimella@broadcom.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>