[S390] ETR support.

This patch adds support for clock synchronization to an external time
reference (ETR). The external time reference sends an oscillator
signal and a synchronization signal every 2^20 microseconds to keep
the TOD clocks of all connected servers in sync. For availability
two ETR units can be connected to a machine. If the clock deviates
for more than the sync-check tolerance all cpus get a machine check
that indicates that the clock is out of sync. For the lovely details
how to get the clock back in sync see the code below.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/smp.c

@@ -457,9 +457,10 @@ int __devinit start_secondary(void *cpuvoid)
         /* Setup the cpu */
         cpu_init();
 	preempt_disable();
-        /* init per CPU timer */
+	/* Enable TOD clock interrupts on the secondary cpu. */
         init_cpu_timer();
 #ifdef CONFIG_VIRT_TIMER
+	/* Enable cpu timer interrupts on the secondary cpu. */
         init_cpu_vtimer();
 #endif
 	/* Enable pfault pseudo page faults on this cpu. */