powerpc/powernv: Add OPAL IPMI interface

Recent OPAL firmare adds a couple of functions to send and receive IPMI
messages:

  https://github.com/open-power/skiboot/commit/b2a374da



This change updates the token list and wrappers to suit, and adds the
platform devices for any IPMI interfaces.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>