target-ppc: Enable DABRX SPR and limit it to <=POWER7

This adds DABRX SPR.

As DABR(X) are present in POWER CPUs till POWER7 only and POWER8 does not
have them (as it implements more powerful facility instead), this limits
DABR/DABRX registration by POWER7 (inclusive).

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