usb: gadget: udc: atmel: Use devm_ioremap_resource()

As devm_ioremap_resource() checks for valid resource,
make use of it instead of testing ourselves. As a bonus
memory region is requested.

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>