watchdog: rza_wdt: Support longer timeouts

The RZ/A2 watchdog timer extends the clock source options in order to
allow for longer timeouts.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>