arm: Simplify CPU counter support

Use the standard ARMv7-M systick module for the ARMv7-M CPU counter
instead of DWT counter since the DWT counter is affected by power saving
states.

Use an inline function for _CPU_Counter_difference() for all ARM BSPs.

Update #3456.

bsps/arm/include/bsp/clock-armv7m.h

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2011, 2018 Sebastian Huber.  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.
+ */
+
+#ifndef BSP_CLOCK_ARMV7M_H
+#define BSP_CLOCK_ARMV7M_H
+
+#include <rtems/score/armv7m.h>
+#include <rtems/timecounter.h>
+
+#include <bsp.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+
+#ifdef ARM_MULTILIB_ARCH_V7M
+
+typedef struct {
+  struct timecounter base;
+  uint32_t ticks;
+} ARMV7M_Timecounter;
+
+extern ARMV7M_Timecounter _ARMV7M_TC;
+
+static inline uint32_t _ARMV7M_Clock_frequency(void)
+{
+#ifdef BSP_ARMV7M_SYSTICK_FREQUENCY
+  return BSP_ARMV7M_SYSTICK_FREQUENCY;
+#else
+  volatile ARMV7M_Systick *systick = _ARMV7M_Systick;
+  return ARMV7M_SYSTICK_CALIB_TENMS_GET(systick->calib) * 100;
+#endif
+}
+
+static uint32_t _ARMV7M_Clock_counter(ARMV7M_Timecounter *tc)
+{
+  volatile ARMV7M_Systick *systick;
+  rtems_interrupt_level level;
+  uint32_t interval;
+  uint32_t counter;
+  uint32_t ticks;
+
+  systick = _ARMV7M_Systick;
+  interval = systick->rvr;
+
+  rtems_interrupt_disable(level);
+  counter = systick->cvr;
+  ticks = tc->ticks;
+
+  if ((systick->csr & ARMV7M_SYSTICK_CSR_COUNTFLAG) != 0) {
+    ticks += interval;
+    tc->ticks = ticks;
+  }
+
+  counter = interval - counter + ticks;
+  rtems_interrupt_enable(level);
+
+  return counter;
+}
+
+#endif /* ARM_MULTILIB_ARCH_V7M */
+
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+
+#endif /* BSP_CLOCK_ARMV7M_H */