dt-bindings: watchdog: airoha: document watchdog for Airoha EN7581

Document watchdog for Airoha EN7581. This SoC implement a simple
watchdog that supports a max timeout of 28 seconds.

The watchdog ticks on half the BUS clock and requires the BUS clock to be
referenced.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20241011104411.28659-1-ansuelsmth@gmail.com


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