MAINTAINERS: Remove machine specific files from ppc TCG CPUs entry

Currently the PowerPC TCG CPUs entry in MAINTAINERS lists all of hw/ppc/
and include/hw/ppc.  Nearly all the files in those places are related to
specific ppc machine types, rather than to the actual CPUs however.  Those
machine types list their own files separately, often overlapping with this.
For greater clarity, remove these misleading entries from the TCG CPUs
stanza, leaving just hw/ppc/ppc.c and hw/ppc/ppc_booke.c which are the only
ones common to a wide range of PPC TCG cpus each.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>