powerpc/fadump: define OPAL register/un-register callback functions

Make OPAL calls to register and un-register with firmware for MPIPL.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821348482.5656.13646250851483648241.stgit@hbathini.in.ibm.com