dt-bindings: usb: bdc: Add Device Tree binding for Broadcom UDC driver

Add Device Tree binding document for Broadcom USB Device
Controller (BDC).

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>