ARM: dts: imx6sx-sabreauto: add wdog external reset

i.MX6SX Sabre Auto board has GPIO1_IO13 pin can be
MUXed as WDOG output to reset PMIC, add this function
support.

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