pmac: build fix

Ensure that BLK_DEV_IDE is built-in before allowing BLK_DEV_IDE_PMAC to
be selected.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Cc: Bret Towe <magnade@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>