phy: Add usb-nop-xceiv support ...

Add a nop usb transcveiver driver. At the moment it does nothing,
so is nothing more than a driver to satisfy the device tree
dependencies. clk / vbus-regulator / vbus-detect-gpio support can be
added later.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>