ipmi: bt: Add ast2600 compatible string

The AST2600 has the same register set as the previous generation SoCs.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Message-Id: <20210903015314.177987-1-joel@jms.id.au>
Signed-off-by: Corey Minyard <cminyard@mvista.com>