usb: host: xhci-dwc3: Add dual role mode support from DT

DWC3 dual role mode is selected using DT "dr_mode"
property. If not found, DWC3 controller is configured
in HOST mode by default

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>