m68k: Add arch/m68k/Kbuild

Use the standard obj-y form to specify the sub-directories under
arch/m68k/. No functional change intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20200526123810.301667-1-masahiroy@kernel.org


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>