usb: dwc2: allow peripheral mode for OTG configuration

Allow device mode in DWC2 driver when device tree select the dr_mode
"peripheral" or "otg".

The device mode is not allowed when dr_mode = "host" in device tree.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Marek Vasut <marex@denx.de>