Some device tree configurations place the generic timer under the root

of the tree and not under simplebus. Update the driver to handle this.

Submitted by:	Julien Grall <julien.grall AT linaro.org>
MFC after:	1 week

sys/arm/arm/generic_timer.c

@@ -353,6 +353,8 @@ static devclass_t arm_tmr_devclass;
 
 EARLY_DRIVER_MODULE(timer, simplebus, arm_tmr_driver, arm_tmr_devclass, 0, 0,
     BUS_PASS_TIMER + BUS_PASS_ORDER_MIDDLE);
+EARLY_DRIVER_MODULE(timer, ofwbus, arm_tmr_driver, arm_tmr_devclass, 0, 0,
+    BUS_PASS_TIMER + BUS_PASS_ORDER_MIDDLE);
 
 void
 DELAY(int usec)