usb: dwc2: Update the gadget driver to use common dwc2_hsotg structure

Adds the gadget data structure and appropriate data structure pointers
to the common dwc2_hsotg data structure. To keep the driver data
dereference code looking clean, the gadget variable declares are only available
for peripheral and dual-role mode. This is needed so that the dwc2_hsotg data
structure can be used by the hcd and gadget drivers.

Updates gadget.c to use the dwc2_hsotg data structure and gadget pointers
that have been moved into the common dwc2_hsotg structure.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>