reset: rzv2h-usb2phy: Add support for VBUS mux controller registration

reset: rzv2h-usb2phy: Add support for VBUS mux controller registration

The RZ/V2H USB2 PHY requires control of the VBUS selection line
(VBENCTL) through a mux controller described in the device tree as
"mux-controller". This change adds support for registering the
rzv2h-usb-vbenctl auxiliary driver during probe.

This enables proper management of USB2.0 VBUS source selection on
platforms using the RZ/V2H SoC.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>