sim: Only print the power state transition warning once

A lot of objects seem to request no-op power transitions which
currently results in large amounts of warnings. These warnings are
benign and risk hiding more serious warnings. Make the warning a
warn_once to prevent console flooding.

Change-Id: I86c74b4224b167f14469250ef86ab69fde7a227e
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/sim/clocked_object.cc

@@ -78,8 +78,8 @@ ClockedObject::pwrState(Enums::PwrState p)
 {
     // Function should ideally be called only when there is a state change
     if (_currPwrState == p) {
-        warn("ClockedObject: Already in the requested power state, request "\
-             "ignored");
+        warn_once("ClockedObject: Already in the requested power state, " \
+                  "request ignored");
         return;
     }