watchdog: imx: Add DT ext-reset handling

The Linux imx2_wdt driver uses a fsl,ext-reset-output boolean in the
device tree to specify whether the board design should use the external
reset instead of the internal reset. Use this boolean to determine which
mode to use rather than using external reset unconditionally.

For the legacy non-DM mode, the external reset is always used in order
to maintain the previous behavior.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>