clock: make get_time_ns() safe to be called without clocksource

make it possible to call get_time_ns() before the clocksource
has been registered. Just return 0 in this case which is still
better than crashing the system.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

common/clock.c

@@ -44,6 +44,9 @@ uint64_t get_time_ns(void)
         uint64_t cycle_now, cycle_delta;
         uint64_t ns_offset;
 
+	if (!cs)
+		return 0;
+
         /* read clocksource: */
 	cycle_now = cs->read() & cs->mask;