ASoC: sun4i-codec: Add support for A31 Line In playback

The A31 integrated codec has a stereo "Line In" input. Add support for
it to the playback paths.

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>