ipmi: bt-bmc: Add ast2500 compatible string

The ast2500 SoCs contain the same IPMI BT device.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>