ipmi: bt: Add ast2600 compatible string

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

Signed-off-by: Joel Stanley <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Corey Minyard <[e-mail]>

Documentation/devicetree/bindings/ipmi/aspeed,ast2400-ibt-bmc.txt

···
 - compatible : should be one of
 	"aspeed,ast2400-ibt-bmc"
 	"aspeed,ast2500-ibt-bmc"
+	"aspeed,ast2600-ibt-bmc"
 - reg: physical address and size of the registers
 
 Optional properties:

drivers/char/ipmi/bt-bmc.c

···
 static const struct of_device_id bt_bmc_match[] = {
 	{ .compatible = "aspeed,ast2400-ibt-bmc" },
 	{ .compatible = "aspeed,ast2500-ibt-bmc" },
+	{ .compatible = "aspeed,ast2600-ibt-bmc" },
 	{ },
 };