imx: imx8mn_var_som: clean up board watchdog code

pinctrl_wdog already marked u-boot,dm-spl, so clean up board code.

The set_wdog_reset() function is not necessary as this is handled by
the imx_watchdog.c driver due to the 'fsl,ext-reset-output' property
being set.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>