ARM: dts: imx: Add WDT reboot bindings on DH i.MX6 DHSOM

Add WDT reboot bindings on DH i.MX6 DHSOM to permit the platform
to reboot via WDT in U-Boot. These are custom U-Boot bindings,
hence they are placed in -u-boot.dtsi .

Reviewed-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Marek Vasut <marex@denx.de>