ide: move legacy ISA/VLB ports handling to ide-legacy.c (v2)

* Move legacy ISA/VLB ports handling from ide-probe.c to ide-legacy.c.

* Add CONFIG_IDE_LEGACY config option to be selected by host drivers
  needing ide-legacy.c.

v2:
Fix CONFIG_IDE_LEGACY not being defined in Kconfig.
(from Takashi Iwai <tiwai@suse.de>)

There should be no functional changes caused by this patch.

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