Add _CPU_Counter_frequency()

Add rtems_counter_frequency() API function.  Use it to initialize the
counter value converter via the new system initialization step
(RTEMS_SYSINIT_CPU_COUNTER).  This decouples the counter implementation
and the counter converter.  It avoids an unnecessary pull in of the
64-bit integer division from libgcc.

Update #3456.

bsps/shared/dev/cpucounter/cpucounterfrequency.c

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2018 embedded brains GmbH.  All rights reserved.
+ *
+ *  embedded brains GmbH
+ *  Dornierstr. 4
+ *  82178 Puchheim
+ *  Germany
+ *  <rtems@embedded-brains.de>
+ *
+ * The license and distribution terms for this file may be
+ * found in the file LICENSE in this distribution or at
+ * http://www.rtems.org/license/LICENSE.
+ */
+
+#include <rtems/score/cpu.h>
+
+uint32_t _CPU_Counter_frequency( void )
+{
+  return 1000000000;
+}