watchdog: renesas_wdt: Add restart handler

On iWave's boards iwg20d and iwg22d the only way to reboot the system is
by means of the watchdog.
This patch adds a restart handler to rwdt_ops, and also makes sure we
keep its priority to the lowest level, in order to not override other
more effective handlers.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>