dt-bindings: phy: renesas,usb2-phy: Document USB VBUS regulator

dt-bindings: phy: renesas,usb2-phy: Document USB VBUS regulator

Document the 'vbus-regulator' child node in the Renesas USB2 PHY binding
to describe the internal USB VBUS regulator.

Require this regulator node on OTG channels to accurately represent
hardware dependencies in the device tree.

Documenting this regulator allows device trees to model the VBUS power
requirements of these SoCs properly.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Link: https://patch.msgid.link/aaa8044283eb736817afd43d4fba3aa93b50b1dd.1766405010.git.tommaso.merciai.xr@bp.renesas.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>