ALSA: Add a note on dependency of RTC stuff

Added a note on the dependency of old RTC stuff, which is exclusive
with the new RTC class drivers.
    http://bugme.linux-foundation.org/show_bug.cgi?id=11430



Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>