usb: phy-zynq: Enable Vbus if port A is detected.

With the current implementation, A-device can only provide Vbus to
B-device through sysfs attribute "a_bus_req".
So, modified the driver to enable Vbus if port A is detected so that
with out userspace help the vbus can be provided to the B-device.

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