imx: ventana: add dt fixup for watchdog external reset

Added removal of the fsl,ext-reset-output property in the wdog node for board
revisions that pre-date the addition of the external watchdog reset signal.

This property is a recent addition to mainline linux kernel in order to
specify that the IMX watchdog external reset should be used instead of the
internal chip-level reset.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>