arm64: dts: meson-axg: s400: add sound card

Add the sound card of the s400. With it the following interface
should be working:
 * Lineout
 * Daugther card speaker 1 (same output as lineout)
 * Linein
 * SPDIF output

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>