sh: Tidy up dependencies for SH-2 build.

SH-2 can presently get in to some pretty bogus states, so
we tidy up the dependencies a bit and get it all building
again.

This gets us a bit closer to a functional allyesconfig
and allmodconfig, though there are still a few things to
fix up.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>