ide: limit legacy VLB host drivers to alpha, x86 and mips

These host drivers indirectly depend on CONFIG_IDE_ARCH_OBSOLETE_DEFAULTS=y
which is defined only on alpha, x86, ia64, m32r, mips and ppc32.

Moreover:

- on ia64 there is no ISA

- m32r is too new for VLB

- on ppc32 ISA is available only on PPC_CHRP (no default IDE ports)
  and PPC_PREP (marked as BROKEN)

[ the common sense tells me that VLB was only used on x86 but there
  are urban legends that one of these host drivers was needed on some
  other arch - thus the extra care ]

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