m68k: merge MMU and non-MMU versions of asm/hardirq.h

The contents of asm/hardirq.h are pretty strait forward for both the
MMU (hardirq_mm.h) and non-MMU (hardirq_no.h) include files. Merge the
two back into a single file.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>