mfd: rn5t618: Register power off callback optionally

Only register power off if the PMIC is defined as system power
controller (see Documentation/devicetree/bindings/power/
power-controller.txt).
Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/rn5t618.c

@@ -102,9 +102,13 @@ static int rn5t618_i2c_probe(struct i2c_client *i2c,
 		return ret;
 	}
 
+	if (of_device_is_system_power_controller(i2c->dev.of_node)) {
 		if (!pm_power_off) {
 			rn5t618_pm_power_off = priv;
 			pm_power_off = rn5t618_power_off;
+		} else {
+			dev_warn(&i2c->dev, "Poweroff callback already assigned\n");
+		}
 	}
 
 	return 0;