dt-bindings: phy: renesas,usb2-phy: Document mux-states property

Some Renesas SoCs, such as RZ/G3E, provide a USB2.0 OTG PHY with
configurable VBUS control through a multiplexed hardware register.
This register allows selecting the VBUS source via a mux control
line exposed by the PHY.

To represent this hardware configuration, support the standard
`mux-states` property in the Renesas USB2 PHY binding.
This allows the DeviceTree to model the VBUS source selection as
a mux, consistent with generic binding conventions.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Link: https://patch.msgid.link/36d448dd10bbb2bbfa5b1b6b6e3fee86c34d01aa.1766405010.git.tommaso.merciai.xr@bp.renesas.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>