usb: dwc2: gadget: add device tree property to enable dma

* Add an of specific function to parse device node properties.
* Enable dma usage only if device tree property 'g_use_dma' is present.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>