dt-bindings: watchdog: factor out RZ/A watchdog

dt-bindings: watchdog: factor out RZ/A watchdog

Renesas created different watchdog IPs but they are all handled in the
same binding documentation. This leads to a lot of conditional handling
which makes it unnecessarily hard to add new items. Factor out the RZ/A
watchdog to make handling easier.

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>