ipmi:ipmb: Add initial support for IPMI over IPMB

This provides access to the management controllers on an IPMB bus to a
device sitting on the IPMB bus.  It also provides slave capability to
respond to received messages on the bus.

Signed-off-by: Corey Minyard <minyard@acm.org>
Tested-by: Andrew Manley <andrew.manley@sealingtech.com>
Reviewed-by: Andrew Manley <andrew.manley@sealingtech.com>