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>