ALSA: doc: ReSTize hdspm.txt

A simple conversion from a plain text file.  Quite a few reformatting
in the end due to the style of the original document.

Put to cards directory.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/cards/index.rst

@@ -14,3 +14,4 @@ Card-Specific Information
    mixart
    bt87x
    maya44
+   hdspm