include/zephyr/drivers/clock_control: return `-ENOSYS` for empty `on`/`off`

There are in-tree clock controllers that don't implement the `on`/`off`
calls, e.g. in Intel Agilex. Let's return `-ENOSYS` when that's the case.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>
(cherry picked from commit 5dbf61aeb8)

include/zephyr/drivers/clock_control.h

@@ -128,6 +128,10 @@ static inline int clock_control_on(const struct device *dev,
 	const struct clock_control_driver_api *api =
 		(const struct clock_control_driver_api *)dev->api;
 
+	if (api->on == NULL) {
+		return -ENOSYS;
+	}
+
 	return api->on(dev, sys);
 }
 
@@ -147,6 +151,10 @@ static inline int clock_control_off(const struct device *dev,
 	const struct clock_control_driver_api *api =
 		(const struct clock_control_driver_api *)dev->api;
 
+	if (api->off == NULL) {
+		return -ENOSYS;
+	}
+
 	return api->off(dev, sys);
 }