Add support for IPMB driver

Support receiving IPMB requests on a Satellite MC from the BMC.
Once a response is ready, this driver will send back a response
to the BMC via the IPMB channel.

Signed-off-by: Asmaa Mnebhi <Asmaa@mellanox.com>
Acked-by:  <vadimp@mellanox.com>
Message-Id: <319690553a0da2a1e80b400941341081b383e5f1.1560192707.git.Asmaa@mellanox.com>
[Move the config option to outside the ipmi msghandler, as it's not
 dependent on that.  Fixed one small whitespace issue.]
Signed-off-by: Corey Minyard <cminyard@mvista.com>