mpc8xx: Only build mpc8xx on arch/ppc

Currently the mpc8xx ide driver will only work on arch/ppc so only
allow it to be built there.  Also, killed a minor include that isn't
actually used by the driver.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>