usb: dwc3: gadget: add a pointer to endpoint registers

By adding a pointer to endpoint registers' base
address, we can avoid using our controller-wide
struct dwc3 pointer for everything. At some point
this will allow us to have per-endpoint locks which
will, in turn, let us queue requests to separate
endpoints in parallel.

Because of this change our debugfs interface and io
accessors need to be changed accordingly.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>