drivers: usb: device: Add support for USB FS on STM32H7.

* Assumes that the USB2_OTG peripheral is being used in FS mode, as on
  the NUCLEO-H743/753.
* Disable ULPI on USB2_OTFG, enable FS clk.
* Enable USB voltage detect and disable USB reg, per STM app notes.

Signed-off-by: Jeremy Wood <jeremy@bcdevices.com>