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/arm/lpc24xx/start/system-clocks.c

@@ -61,8 +61,11 @@ void lpc24xx_timer_initialize(void)
 
   /* Start timer */
   T1TCR = TCR_EN;
+}
 
-  rtems_counter_initialize_converter(LPC24XX_PCLK);
+uint32_t _CPU_Counter_frequency(void)
+{
+  return LPC24XX_PCLK;
 }
 
 CPU_Counter_ticks _CPU_Counter_read(void)