The watchdog can either reset only the SoC or assert the WDOG_B
output signal instead. On some boards it's necessary to use the
external WDOG_B output to make sure that external devices like the
PMIC are also properly resetted. This has been fixed in the Linux
driver which honours a fsl,ext-reset-output device tree property
to select between both ways. Do the same in the barebox driver.

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