ALSA: core: Manage asound root directory with snd_info_entry

Using snd_info_entry for /proc/asound root makes easier to release the
all children, too.  Further cleanups will follow.

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