usb: dwc3: Keeping 'resource' related code together

Putting together the code related to getting the 'IORESOURCE_MEM'
and assigning the same to dwc->xhci_resources, for increasing
the readability.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>