bsps: Move shared CPU counter support to bsps

This patch is a part of the BSP source reorganization.

Update #3285.

bsps/shared/dev/cpucounter/cpucounterdiff.c

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2014 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>
+
+CPU_Counter_ticks _CPU_Counter_difference(
+  CPU_Counter_ticks second,
+  CPU_Counter_ticks first
+)
+{
+  return second - first;
+}