ARM: dts: imx6sll-evk: use WDOG_B pin reset

i.MX6SLL EVK board has WDOG_B pin connected to the PMIC;

Add the WDOG_B pinctrl entry and 'fsl,ext-reset-output'
property to wdog node to let watchdog trigger a system
POR reset via the PMIC.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>