ALSA: doc: ReSTize VIA82xx-mixer.txt

Another simple conversion from a plain text file.
Put to cards directory.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/VIA82xx-mixer.txt

-
-				VIA82xx mixer
-				=============
-
-On many VIA82xx boards, the 'Input Source Select' mixer control does not work.
-Setting it to 'Input2' on such boards will cause recording to hang, or fail
-with EIO (input/output error) via OSS emulation.  This control should be left
-at 'Input1' for such cards.

Documentation/sound/cards/index.rst

@@ -9,3 +9,4 @@ Card-Specific Information
    sb-live-mixer
    audigy-mixer
    emu10k1-jack
+   via82xx-mixer

Documentation/sound/cards/via82xx-mixer.rst

+=============
+VIA82xx mixer
+=============
+
+On many VIA82xx boards, the ``Input Source Select`` mixer control does not work.
+Setting it to ``Input2`` on such boards will cause recording to hang, or fail
+with EIO (input/output error) via OSS emulation.  This control should be left
+at ``Input1`` for such cards.