usb: bdc: Add support for USB phy

If a phy is specified in the device tree node, get it and use it.
This was based on a patch by:
"Srinath Mannam <srinath.mannam@broadcom.com>"

Signed-off-by: Al Cooper <alcooperx@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>