From: Geert Uytterhoeven <geert@linux-m68k.org>

Update M68k configuration logic:
  - Introduce CONFIG_MMU_MOTOROLA and CONFIG_MMU_SUN3
  - Move Sun-3 config selection to the top since all other platforms conflict
    with it
  - Sun-3 implies MMU_SUN3
  - All other platforms imply MMU_MOTOROLA
  - Sun-3 implies M68020
  - Sun-3x implies M68030
  - Sun-3 kernels cannot support 68030, 68040, or 68060