m68k: merge the MMU and non-MMU versions of the arch dma code

The majority of the m68k architecture dma code is the same, so merge the
current separated files dma_no.c and dma_mm.c back into a single dma.c

The main alloc and free routines are a little different, so we keep a
single #ifdef based on CONFIG_MMU for them. All the other support functions
are now identical.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>