usb: phy: Fix USB_ULPI_VIEWPORT Kconfig dependency

USB_ULPI_VIEWPORT didn't depend on USB_ULPI, while USB_ULPI
is using non user selectable USB_ULPI_VIEWPORT. Fix this.

Signed-off-by: Ivan T. Ivanov <ivan.ivanov@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>