powerpc/pseries: Remove MPIC from pseries kexec

MPIC was only used by Power3 which is now unsupported, so remove MPIC
code. XICS is now the only supported interrupt controller for
pSeries so do some cleanups too.

Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>