watchdog: renesas_wdt: add grace period before rebooting

arm64 does not have a grace period after calling reset handlers. It is
rightfully assumed that watchdog drivers should wait because they know
the time needed. Implement this for the Renesas watchdog driver.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Guenter Roeck <linux@oeck-us.net>
Link: https://lore.kernel.org/r/20210118094558.36814-1-wsa+renesas@sang-engineering.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>