dt-bindings: musb: mpfs: add ULPI external vbus support

Add a property to allow configuring the musb controller on PolarFire SoC
to use an external vbus for ULPI PHYs.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240531-fountain-plating-1d3739422a26@wendy


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>