dt-bindings: watchdog: renesas-wdt: Add support for R7S9210

Document support for RZ/A2

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
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>