powerpc/sprs: Test POWER9 specific registers

Most of the SPRS are the same as on POWER8, so we can re-use
the PowerISA 2.07 functions and simply amend the additional
registers afterwards.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>