sh: hd64461.h cleanup and added comments.

Now that we dont have PIO mapping anymore we need to make sure we
got the correct value in our headers. Some well needed comments
have also been added.

Signed-off-by: Kristoffer Ericson <kristoffer.ericson@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>