phy: stm32: manage optional vbus regulator on phy_power_on/off

This patch adds support for optional vbus regulator.
It is managed on phy_power_on/off calls and may be needed for host mode.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20210517120821.26466-3-amelie.delaunay@foss.st.com


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