watchdog: Clean up defaults for imx_watchdog / ulp_wdog

In imx_watchdog, clean up the comment to just note the range now, as we
do not need to set the default here as Kconfig does this for us. For
ulp_wdog, set the default value via Kconfig instead.

Cc: Stefan Roese <sr@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefan Roese <sr@denx.de>