ide-pmac: add ->init_dev method (take 4)

v2/3:
* Build fixes from Stephen Rothwell.

v4:
* Fix ->noprobe setting. (From Benjamin Herrenschmidt)

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>