ARM: dts: silk: Enable watchdog support

Enable the watchdog, so the board can be restarted by a watchdog
timeout.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>