ARM: dts: imx6: rdu2: enable WDOG1

Enable the i.MX6 WDOG1 internal watchdog for warm reboots. This allows
to issue emergency restarts without clearing the RAM, so collecting oops
logs from ramoops pstore in barebox becomes feasible.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>