m68knommu: fix build when CPU is not coldfire

Commit dd1cb3a7 [merge MMU and non-MMU
versions of mm/init.c] unified mm/init.c for both MMU and non-MMU m68k
platforms. However, it broke when we build a non-MMU M68K Classic CPU kernel.

This fix builds a section that came from the MMU version only when we are
building a MMU kernel.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>