Add a cputable entry for the POWER6 processor.

The SIHV and SIPR bits in the mmcra have moved in POWER6, so disable
support for that until oprofile is fixed.

Also tell firmware that we know about POWER6.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>