zynq usb: Allow use of external VBUS regulator

Add support for driving an external VBUS regulator. This makes it possible
to operate the USB in OTG mode without requiring programmable logic to drive
the VBUS controls, such as a MIO GPIO pin or external I2C regulator.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Reviewed-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>