m68k: make interrupt definitions conditional on correct CPU types

The interrupt handling support defines and code is not so much conditional
on an MMU being present (CONFIG_MMU), as it is on which type of CPU we are
building for. So make the code conditional on the CPU types instead. The
current irq.h is mostly specific to the interrupt code for the 680x0 CPUs,
so it should only be used for them.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Matt Waddel <mwaddel@yahoo.com>
Acked-by: Kurt Mahan <kmahan@xmission.com>