drivers/char/ipmi: Add powernv IPMI driver

This change adds an initial IPMI driver for powerpc OPAL firmware. The
interface is exposed entirely through firmware: we have two functions to
send and receive IPMI messages, and an interrupt notification from the
firmware to signify that a message is available.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>