sh: Disable SCIF2 on the SH-X3 proto CPU.

SCIF2 and the FPU exceptions happen to share vector numbers, one in
EXPEVT and the other in INTEVT. This is a violation of the interface and
should have never made it in to silicon. On top of that, the demux hack
that was added for special dispatch is rather error prone, and introduces
more problems than it solves. Kill all of it off, and just refuse to deal
with SCIF2 outright.

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