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