arm64: dts: freescale: tqma9352: Fix watchdog reset

On the tqma9352 the board is reset through an external PMIC, so
set the fsl,ext-reset-output property to enable triggering the
output pin on a watchdog trigger.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>