usb: dwc2: gadget: Add descriptor DMA parameter

Add a parameter for descriptor DMA and set it based on hardware
capabilities. This won't actually be used by the gadget until later,
when the descriptor DMA code is in place.

Signed-off-by: Vahram Aharonyan <vahrama@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>