[IA64] Fix CONFIG_PRINTK_TIME

There were two problems with enabling the PRINTK_TIME config
option:
1) The first calls to printk() occur before per-cpu data virtual
address is pinned into the TLB, so sched_clock() can fault.
2) sched_clock() is based on ar.itc, which may not be synchronized
across cpus.

Ken Chen started this patch, Tony Luck tinkered with it, and Jes
Sorensen perfected it.

Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/kernel/time.c

@@ -278,3 +278,30 @@ udelay (unsigned long usecs)
 	}
 }
 EXPORT_SYMBOL(udelay);
+
+static unsigned long long ia64_itc_printk_clock(void)
+{
+	if (ia64_get_kr(IA64_KR_PER_CPU_DATA))
+		return sched_clock();
+	return 0;
+}
+
+static unsigned long long ia64_default_printk_clock(void)
+{
+	return (unsigned long long)(jiffies_64 - INITIAL_JIFFIES) *
+		(1000000000/HZ);
+}
+
+unsigned long long (*ia64_printk_clock)(void) = &ia64_default_printk_clock;
+
+unsigned long long printk_clock(void)
+{
+	return ia64_printk_clock();
+}
+
+void __init
+ia64_setup_printk_clock(void)
+{
+	if (!(sal_platform_features & IA64_SAL_PLATFORM_FEATURE_ITC_DRIFT))
+		ia64_printk_clock = ia64_itc_printk_clock;
+}