ASoC: sun4i-codec: Merge sun4i_codec_left_mixer_controls and sun4i_codec_right_mixer_controls into sun4i_codec_mixer_controls

Since it's now possible to have a DAPM mixer control with multiple
channels, use it to cut down the total number of controls.

Keep "Left Mixer Left DAC Playback Switch" and "Right Mixer Right DAC
Playback Switch" name & layout the same as before for compatibility.

Signed-off-by: Danny Milosavljevic <dannym@scratchpost.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>