ARM: dts: sun9i: Add device node for watchdog

On A80 there are 2 watchdogs, one in the main block, and one in the
R (special) block. We do not have information on the R block watchdog,
other than the register layout is the same, and the interrupt number.
Both are able to reset the whole system.

Add the main watchdog, in case the R block is used for special purposes
like running an RTOS.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>