dsp: add option to swap left and right channels

Change-Id: Id4b518638436576cfb5e747548f10ece6e58eba0

manual/configure_rockbox/sound_settings.tex

@@ -178,6 +178,9 @@ change to customise your listening experience.
           to make the singer sound centrally placed, this often (but not
           always) has the effect of removing the voice track from a song. This
           setting also very often has other undesirable effects on the sound.
+      \item[Swap Left \& Right.]
+          Plays the left channel in the right stereo channel, and the right
+          channel in the left stereo channel.
   \end{description}
 
 \section{Stereo Width}