power_domain: gpio: only run actions for children on domain

Only run the `PM_DEVICE_ACTION_TURN_ON` and `PM_DEVICE_ACTION_TURN_OFF`
actions for child devices that have refered to the domain via the
`power-domain` property.

This prevents multiple actions being run for devices that refer to
several power domains, e.g.

```
	test_dev: test_dev {
		compatible = "test-device-pm";
		status = "okay";
		power-domain = <&test_reg_1>;
		alternate-domain = <&test_reg_chained>;
	};
```

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>