powernv: Don't test POWER9 CPU yet

powernv: Don't test POWER9 CPU yet

A couple of tests for the work-in-progress 'powernv' machine type attempt
to test on POWER9 CPUs.  However the POWER9 CPU support is incomplete and
this doesn't really work.  In particular the firmware image we have
currently assumes the presence of the SDR1 register, which no longer exists
on POWER9.  We only got away with this so far, because of a different bug
which added SDR1 to POWER9 even though it shouldn't be there.

For now, remove POWER9 testing of powernv, POWER8 testing will do for now
until the POWER9 support is more complete.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>