ASoC: sunxi: add support for the on-chip codec on early Allwinner SoCs

The sun4i, sun5i and sun7i SoC families have a built-in codec, capable
of both audio capture and playback.

While this is called a codec by Allwinner, it really is an in-SoC
combination of a codec and a DAI, with its own DAC/ADC and amplifiers
in a single memory-mapped controller.

The capture part has been left out for now, and will be added eventually.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>