ide: rework PowerMac media-bay support (take 2)

Rework PowerMac media-bay support in such way that instead of
un/registering the IDE interface we un/register IDE devices:

* Add ide_port_scan() helper for probing+registerering devices on a port.

* Rename ide_port_unregister_devices() to __ide_port_unregister_devices().

* Add ide_port_unregister_devices() helper for unregistering devices on a port.

* Add 'ide_hwif_t *cd_port' to 'struct media_bay_info', pass 'hwif' instead
  of hwif->index to media_bay_set_ide_infos() and use it to setup 'cd_port'.

* Use ide_port_unregister_devices() instead of ide_unregister()
  and ide_port_scan() instead of ide_register_hw() in media_bay_step().

* Unexport ide_register_hw() and make it static.

v2:
* Fix build by adding <linux/ide.h> include to <asm-powerpc/mediabay.h>.
  (Reported by Michael/Kamalesh/Andrew).

Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Cc: Michael Ellerman <michael@ellerman.id.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>