ARM: dts: imx6sx-sdb: Use WDOG_B pin reset

imx6sx-sdb has WDOG1_B pin connected to the PMIC.

Pass the 'fsl,ext-reset-output' property so that the watchdog
can trigger a system POR reset via the PMIC.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>