ipmi: kcs: aspeed: Add AST2600 compatible string

The AST2600 is already described in the bindings, but the driver never
gained a compatible string.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Message-Id: <20220221070351.121905-1-joel@jms.id.au>
Signed-off-by: Corey Minyard <cminyard@mvista.com>