ARM: dts: r7s72100: Add watchdog timer

Add watchdog timer support for RZ/A1.
For the RZ/A1, the only way to do a reset is to overflow the WDT, so this
is useful even if you don't need the watchdog functionality.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>