ide-pmac: dynamically allocate struct pmac_ide_hwif instances (take 2)

* Dynamically allocate struct pmac_ide_hwif instances in pmac_ide_macio_attach()
  and pmac_ide_pci_attach(), then remove no longer needed pmac_ide[].

v2:
* Build fix from Kamalesh Babulal.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>