ARM: dts: tango4: Import watchdog node

The device driver was added in v4.5 by commit dca536c4


("watchdog: add support for Sigma Designs SMP86xx/SMP87xx")

Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>