usb: dwc2: gadget: Enable descriptor DMA mode

Add DCFG register field macro for descriptor DMA mode and update core
initialization routine to set that bit accordingly.

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>