dt-bindings: watchdog: aspeed: External reset signal properties

For the AST2500 and compatible watchdog controllers the external reset
signal can be configured for push-pull or open-drain drive types, and in
the case of push-pull driving, active low or high.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>