usb: dwc2: convert driver to DM_USB_GADGET

Minimal conversion to driver model by using the uclass
UCLASS_USB_GADGET_GENERIC based on:
- reset uclass
- clock uclass
- generic uclass.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>