rtc: hym8563: make the irq optional

Sometimes the irq line is not connected to any soc-pin. This does
not hinder basic timekeeping functionality of the rtc, so probe
should not fail in this case.

Signed-off-by: Heiko Stuebner <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>

Documentation/devicetree/bindings/rtc/haoyu,hym8563.txt

···
 Required properties:
 - compatible: should be: "haoyu,hym8563"
 - reg: i2c address
-- interrupts: rtc alarm/event interrupt
 - #clock-cells: the value should be 0
 
 Optional properties:
 - clock-output-names: From common clock binding
+- interrupts: rtc alarm/event interrupt
 
 Example:
 

drivers/rtc/rtc-hym8563.c

···
 		return ret;
 	}
 
-	ret = devm_request_threaded_irq(&client->dev, client->irq,
-					NULL, hym8563_irq,
-					IRQF_TRIGGER_LOW | IRQF_ONESHOT,
-					client->name, hym8563);
-	if (ret < 0) {
-		dev_err(&client->dev, "irq %d request failed, %d\n",
-			client->irq, ret);
-		return ret;
+	if (client->irq > 0) {
+		ret = devm_request_threaded_irq(&client->dev, client->irq,
+						NULL, hym8563_irq,
+						IRQF_TRIGGER_LOW | IRQF_ONESHOT,
+						client->name, hym8563);
+		if (ret < 0) {
+			dev_err(&client->dev, "irq %d request failed, %d\n",
+				client->irq, ret);
+			return ret;
+		}
 	}
 
 	/* check state of calendar information */