m68k: Add multi_defconfig

Add multi_defconfig, to build a kernel for all supported m68k platforms,
excluding Sun 3 (Sun 3 kernels are incompatible with all other m68k platforms)

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>