phy: stm32: usbphyc: 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: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>