drivers: timer: sys_clock: return -ENOSYS if not implemented

-ENOSYS should be returned if the operation is not implemented. This
issue was causing some PM tests to fail, as -ENOSYS was expected.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/drivers/timer/sys_clock_init.c b/drivers/timer/sys_clock_init.c
index 62ddcfe..c994525 100644
--- a/drivers/timer/sys_clock_init.c
+++ b/drivers/timer/sys_clock_init.c
@@ -34,7 +34,7 @@
 			       uint32_t ctrl_command,
 			       uint32_t *context, pm_device_cb cb, void *arg)
 {
-	return -ENOTSUP;
+	return -ENOSYS;
 }
 
 void __weak sys_clock_set_timeout(int32_t ticks, bool idle)