On PPC, the default PAGE_SIZE is 64kb. Unfortunately, the hardware
alignments don't match here: There are RAM and MMIO regions within
a single page when it's 64kb in size.

So the only way out for now is to tell the user that he should use 4k
PAGE_SIZE.

This patch gives the user a hint on that, telling him that failing to
register a prefix slot is most likely to be caused by mismatching PAGE_SIZE.

This way it's also more future-proof, as bigger PAGE_SIZE can easily be
supported by other machines then, as long as they stick to 64kb granularities.

Signed-off-by: Alexander Graf <agraf@suse.de>