drivers: usb: add common DWC2 register header

This header is based on drivers/usb/device/usb_dw_registers.h and
describes registers of the DWC2 controllers IP and is intended for use
in both device controller drivers and a host controller driver. The
difference to usb_dw_registers.h is that this header does not confuse
offsets with bit positions, contains all the definitions required for
device mode, has register and bit field names identical to the databook
and no annoying underscores.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>