target-ppc: Add "compat" CPU option

PowerISA defines a compatibility mode for server POWERPC CPUs which
is supported by the PCR special register which is hypervisor privileged.
To support this mode for guests, SPAPR defines a set of virtual PVRs,
one per PowerISA spec version. When a hypervisor needs a guest to work in
a compatibility mode, it puts a virtual PVR value into @cpu-version
property of a CPU node.

This introduces a "compat" CPU option which defines maximal compatibility
mode enabled. The supported modes are power6/power7/power8.

This does not change the existing behaviour, new property will be used
by next patches.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
