usb: dwc3: omap: distinguish between SW and HW modes

The OMAP wrapper allows us to either control internal
OTG signals via SW or HW. Different boards might wish
to use one or the other mode of operation. Let's have
have that information passed via platform_data for now.

After DT conversion is finished for OMAP, we can easily
convert this to a DT attribute.

Signed-off-by: Felipe Balbi <balbi@ti.com>