watchdog: aspeed: Remove specific reference to AST2400 in Kconfig

The driver also supports the watchdog in the AST25xx series, and
may work on earlier SoCs as well.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.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@iguana.be>