Documentation: usb: phy: nop: Fix the description of 'vcc-supply'

Since bd27fa44 ("usb: phy: generic: Don't use regulator framework for
RESET line") we no longer model the reset line as a regulator supply, so
adapt the documentation accordingly.

Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>