usb: phy: rename nop_usb_xceiv => usb_phy_gen_xceiv

The "nop" driver isn't a do-nothing-stub but supports a couple functions
like clock on/off or is able to use a voltage regulator. This patch
simply renames the driver to "generic" since it is easy possible to
extend it by a simple function istead of writing a complete driver.

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