usb: dwc2: add "u-boot,force-vbus-detection" for stm32

On some board, the ID pin is not connected so the B session must be
overridden with "u-boot,force_b_session_valid" but the VBus sensing
must continue to be handle.

To managed it, this patch adds a new DT field
"u-boot,force-vbus-detection" to use with "u-boot,force_b_session_valid"

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>