m68knommu: use asm-generic/mmu.h for nommu setups

The nommu case defines its own local mm_context_t structure. There is
nothing special or different about the m68knommu version of this and it
can easily use the common asm-generic version.

Remove the local mmu_context struct and include the asm-generic version
instead. This will also make it easier to support ELF format executables
in the future (since the asm-generic version has support for this
already).

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