USBD add core-power and Vbus handle
Created by: alexrayne
here provided enhances on usbd-driver for:
- Vbus sensing - <usbd_is_vbus> method and callback. This helps to detect cable plug in/out, or it can be USBpowering detection
- USBcore power/clock domain control - <usbd_enable> method and <power_control> backend implementation for STM32F4xxx. This intends to power-saving by usb-core and PHY stops.