usb: phy: generic: add vbus support

Add support for vbus detection and power supply. This code is more or
less stolen from phy-gpio-vbus-usb.c, and aims at providing a detection
mechanism for VBus (ie. usb cable plug) based on a GPIO line, and a
power supply activation which draws current from the VBus.

[ balbi@ti.com : fix build break ]

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Felipe Balbi <balbi@ti.com>