ASoC: sun4i-codec: Add support for A31 ADC capture path

The A31's internal codec capture path has a mixer in front of the ADC
for each channel, capable of selecting various inputs, including
microphones, line in, phone in, and the main output mixer.

This patch adds the various controls, widgets and routes needed for
audio capture from the already supported inputs on the A31.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>