ALSA: Enable build with m68k

By some reason in the ancient history, we disabled the build of ALSA
drivers for m68k.  Since we'd like to move sound/oss/dmasound stuff
into ALSA for the complete drop of the legacy OSS stuff, let's try to
start building with m68k.

Signed-off-by: Takashi Iwai <tiwai@suse.de>