include/timer.h: Guard `timer_monotonic_get()` calls by `CONFIG_HAVE_MONOTONIC_TIMER`

Some platforms do not have `timer_monotonic_get()` implemented. So only
call `timer_monotonic_get()` if `CONFIG_HAVE_MONOTONIC_TIMER` is
selected and set the times to 0 otherwise.

Change-Id: If9cba4c0c17a7011aa357079d8fdd0aa47ad1b66
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/12105
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
diff --git a/src/include/timer.h b/src/include/timer.h
index aa5746d..691d6fb 100644
--- a/src/include/timer.h
+++ b/src/include/timer.h
@@ -130,7 +130,11 @@
 
 static inline void stopwatch_init(struct stopwatch *sw)
 {
-	timer_monotonic_get(&sw->start);
+	if (IS_ENABLED(CONFIG_HAVE_MONOTONIC_TIMER))
+		timer_monotonic_get(&sw->start);
+	else
+		sw->start.microseconds = 0;
+
 	sw->current = sw->expires = sw->start;
 }
 
@@ -150,7 +154,10 @@
  */
 static inline void stopwatch_tick(struct stopwatch *sw)
 {
-	timer_monotonic_get(&sw->current);
+	if (IS_ENABLED(CONFIG_HAVE_MONOTONIC_TIMER))
+		timer_monotonic_get(&sw->current);
+	else
+		sw->current.microseconds = 0;
 }
 
 /*