[POWERPC] iSeries: Move viodasd probing

This way we only have entries in the device tree for disks that actually
exist.  A slight complication is that disks may be attached to LPARs
at runtime.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>