ipmi/bt-bmc: change compatible node to 'aspeed, ast2400-ibt-bmc'

The Aspeed SoCs have two BT interfaces : one is IPMI compliant and the
other is H8S/2168 compliant.

The current ipmi/bt-bmc driver implements the IPMI version and we
should reflect its nature in the compatible node name using
'aspeed,ast2400-ibt-bmc' instead of 'aspeed,ast2400-bt-bmc'. The
latter should be used for a H8S interface driver if it is implemented
one day.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Olof Johansson <olof@lixom.net>