[ARM] Provide basic printk_clock() implementation

Current sched_clock() implementations on ARM cause unbootable kernels
with PRINTK_TIME support enabled.  To avoid this, provide a basic
printk_clock() implementation which avoids sched_clock() being called
before the page tables have been set up.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/time.c

@@ -87,6 +87,17 @@ unsigned long long __attribute__((weak)) sched_clock(void)
 	return (unsigned long long)jiffies * (1000000000 / HZ);
 }
 
+/*
+ * An implementation of printk_clock() independent from
+ * sched_clock().  This avoids non-bootable kernels when
+ * printk_clock is enabled.
+ */
+unsigned long long printk_clock(void)
+{
+	return (unsigned long long)(jiffies - INITIAL_JIFFIES) *
+			(1000000000 / HZ);
+}
+
 static unsigned long next_rtc_update;
 
 /*