usb: renesas_usbhs: Allow an OTG PHY driver to provide VBUS

These changes allow a PHY driver to trigger a VBUS interrupt and
to provide the value of VBUS.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>