usb: omap: Move the usb phy code to the usb/phy directory

Moving the usb/phy code from xhci-omap to the usb/phy directory
and moving the associated phy code over to the new file.

Newer TI processors adding xHCI support will have different PHY configurations
so therefore abstracting this code away will prevent messing around with the
xhci-omap file itself.

Signed-off-by: Dan Murphy <dmurphy@ti.com>