powerpc/vio: Add modalias support

BenH: Added to vio_cmo_dev_attrs as well

Provide a modalias entry for VIO devices in sysfs.  I believe
this was another initrd generation bugfix for anaconda.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>