Associate the swnode of the GPIO device's (which is the interrupt
controller here) with the irq domain. Otherwise the interrupt-controller
device attribute is a no-op.

Fixes: cb8c474e ("gpio: sim: new testing module")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>