ide: add ide_device_add()

* Add ide_device_add() helper and convert host drivers to use it
  instead of open-coded variants.

* Make ide_pci_setup_ports() and do_ide_setup_pci_device()
  take 'u8 *idx' argument instead of 'ata_index_t *index'.

* Remove no longer needed ata_index_t.

* Unexport probe_hwif_init() and make it static.

* Unexport ide_proc_register_port().

There should be no functionality changes caused by this patch
(sgiioc4.c: ide_proc_register_port() requires hwif->present
 to be set and it won't be set if probe_hwif_init() fails).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>