button: qcom-pmic: demote "unknown button" message to debug

This message isn't an error (there can be a watchdog subnode for example)
but it shouldn't be printed unless this driver is being debugged. Demote
it to a debug print.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>

drivers/button/button-qcom-pmic.c

@@ -133,7 +133,7 @@ static int button_qcom_pmic_bind(struct udevice *parent)
 		} else if (NODE_IS_RESIN(node)) {
 			uc_plat->label = "vol_down";
 		} else {
-			printf("Unknown button node '%s' should be 'pwrkey' or 'resin'\n",
+			debug("Unknown button node '%s' should be 'pwrkey' or 'resin'\n",
 			       ofnode_get_name(node));
 			device_unbind(dev);
 		}