usb: phy: Add AM335x PHY driver

usb: phy: Add AM335x PHY driver

This driver is a redo of my earlier attempt. It uses parts of the
generic PHY driver and uses the new control driver for the register
the phy needs to power on/off the phy. It also enables easy access for
the wakeup register which is not yet implemented.
The difference between the omap attempt is:
- no static holding variable
- one global visible function which exports a struct with callbacks to
  access the "control" registers.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>