ASoC: doc: ReSTize DPCM.txt

A simple conversion from a plain text file.
The file name was renamed to lower letters to align with others.
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/soc/index.rst

@@ -16,3 +16,4 @@ The documentation is spilt into the following sections:-
    pops-clicks
    clocking
    jack
+   dpcm