MAINTAINERS: Update powerpc KVM entry

Paul is no longer handling patches for kvmppc.

Instead we'll treat them as regular powerpc patches, taking them via the
powerpc tree, using the topic/ppc-kvm branch when necessary.

Also drop the web reference, it doesn't have any information
specifically relevant to powerpc KVM.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211027061646.540708-1-mpe@ellerman.id.au