cc26xx - implemented watchdog_stop
cc26xx - implemented watchdog_stop

Also modified watchdog_start so that if we stop and start again the
watchdog timeout will be reset (by calling watchdog_periodic).

MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,arurke/contiki