KVM: PPC: Update MAINTAINERS

Michael is merging KVM PPC patches via the powerpc tree and KVM topic
branches. He doesn't necessarily have time to be across all of KVM so
is reluctant to call himself maintainer, but for the mechanics of how
patches flow upstream, it is maintained and does make sense to have
some contact people in MAINTAINERS.

So add Michael Ellerman as KVM PPC maintainer and myself as reviewer.
Split out the subarchs that don't get so much attention.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230608024504.58189-1-npiggin@gmail.com