ppc: remove ppc_ide_md

* Add special cases for pplus and prep to ide_default_{irq,io_base}()
  (+ FIXMEs about the need to use IDE platform host driver instead).

* Remove no longer needed ppc_ide_md and struct ide_machdep_calls.

* Then remove <linux/ide.h> include from:
  - arch/powerpc/kernel/setup_32.c
  - arch/ppc/kernel/ppc_ksyms.c
  - arch/ppc/kernel/setup.c
  - arch/ppc/platforms/pplus.c
  - arch/ppc/platforms/prep_setup.c

There should be no functional changes caused by this patch.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>