watchdog: Add Toshiba Visconti watchdog driver

Add the watchdog driver for Toshiba Visconti series.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Reviewed-by: Punit Agrawal <punit1.agrawal@toshiba.co.jp>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20201005023012.603026-3-nobuhiro1.iwamatsu@toshiba.co.jp
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>