target-ppc: Add more stubs for POWER7 PMU registers

In addition to the performance monitor registers found on nearly all
6xx chips, the POWER7 has two additional counters (PMC5 & PMC6) and an
extra control register (MMCRA).  This patch adds stub support for them to
qemu - the registers won't do anything, but with this change won't cause
illegal instruction traps accessing them.  They're also registered with
their ONE_REG ids, so their value will be kept in sync with KVM where
appropriate.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
