[mac] add enhanced frame pending (#4334)

This commit adds enhanced frame pending feature of Thread 1.2. This
requires the platform radio driver to correctly set frame pending bit
for Data frames and MAC Data Request frames.

tests/scripts/thread-cert/simulator.py

@@ -107,6 +107,9 @@ class RealTime(BaseSimulator):
         self.commissioning_messages[nodeid] = []
         return ret
 
+    def now(self):
+        return time.time()
+
     def go(self, duration, nodeid=None):
         time.sleep(duration)
 
@@ -456,6 +459,9 @@ class VirtualTime(BaseSimulator):
             self.receive_events()
         self.awake_devices.clear()
 
+    def now(self):
+        return self.current_time / 1000000
+
     def go(self, duration, nodeid=None):
         assert self.current_time == self._pause_time
         duration = int(duration) * 1000000