watchdog: aspeed: Add support for AST2600

The ast2600 can be supported by the same code as the ast2500.

Signed-off-by: Ryan Chen <ryan_chen@aspeedtech.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20190819051738.17370-3-joel@jms.id.au
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>