powerpc/e500: split CPU_FTRS_ALWAYS/CPU_FTRS_POSSIBLE

Split e500 (v1/v2) and e500mc/e5500 to allow optimization of feature
checks that differ between the two.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>