Kinetis: Add atomic and non-atomic HRT calls

This reduces interrupt load significantly.

src/drivers/kinetis/drv_hrt.c

@@ -599,6 +599,17 @@ abstime_to_ts(struct timespec *ts, hrt_abstime abstime)
  */
 hrt_abstime
 hrt_elapsed_time(const volatile hrt_abstime *then)
+{
+	hrt_abstime delta = hrt_absolute_time() - *then;
+
+	return delta;
+}
+
+/**
+ * Compare a time value with the current time as atomic operation.
+ */
+hrt_abstime
+hrt_elapsed_time_atomic(const volatile hrt_abstime *then)
 {
 	irqstate_t flags = px4_enter_critical_section();