[ARM] Allow sub-architectures to provide their own sched_clock()

arch/arm/kernel/time.c

@@ -72,8 +72,10 @@ unsigned long (*gettimeoffset)(void) = dummy_gettimeoffset;
 
 /*
  * Scheduler clock - returns current time in nanosec units.
+ * This is the default implementation.  Sub-architecture
+ * implementations can override this.
  */
-unsigned long long sched_clock(void)
+unsigned long long __attribute__((weak)) sched_clock(void)
 {
 	return (unsigned long long)jiffies * (1000000000 / HZ);
 }