dt-bindings: phy: add vbus-supply optional property to phy-stm32-usbphyc

This patch adds vbus-supply optional property to phy sub-nodes using
connector node.
A regulator for USB VBUS may be needed for host mode.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20210517120821.26466-2-amelie.delaunay@foss.st.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>