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

Documentation/devicetree/bindings/usb/microchip,mpfs-musb.yaml

@@ -34,6 +34,13 @@ properties:
   clocks:
     maxItems: 1
 
+  microchip,ext-vbus-drv:
+    description:
+      Some ULPI USB PHYs do not support an internal VBUS supply and driving
+      the CPEN pin requires the configuration of the UPLI_USE__EXTVBUS
+      bit in ULPI_BUSCONTROL.
+    $ref: /schemas/types.yaml#/definitions/flag
+
 required:
   - compatible
   - reg