usb: phy: nop: device tree documentation for vbus

Enhance the phy documentation by adding 2 new optional bindings :
 - the vbus gpio, which detects usb insertion
 - the vbus regulator, which provides current drawn from the usb cable

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