Make sound support modular to reduce kernel size (-571 KiB):

       text    data     bss     dec     hex   filename
    12990441 1438820  342552 14771813  e16665 vmlinux.before
    12451039 1396476  339800 14187315  d87b33 vmlinux.after
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NArnd Bergmann <arnd@arndb.de>