usb: dwc2: add support for host mode external vbus supply

usb: dwc2: add support for host mode external vbus supply

This patch adds a way to enable an external vbus supply in host mode,
when dwc2 drvvbus signal is not used.

This patch is very similar to the one done in U-Boot dwc2 driver [1]. It
also adds dynamic vbus supply management depending on the role and state
of the core.

[1] https://lists.denx.de/pipermail/u-boot/2017-March/283434.html

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>