nrf5: Refresh the watchdog while flashing

nrf5: Refresh the watchdog while flashing

If watchdog is enabled, there's no way we can disable it while the flashing firmware
is running. (Halt disables it, but software reset doesn't.) So let's have the flashing
firmware refresh the watchdog regularly, in case it has been enabled by previously
running software. Failure to do so could lead to a watchdog reset in the middle of
the chip bieng programmed.

Change-Id: I79d41593948aae0080480e891552e1c2ee3ccbd0
Signed-off-by: Aurélien Martin <martaurel@gmail.com>
Reviewed-on: http://openocd.zylin.com/5266
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>