ASoC: meson: axg: extract sound card utils

This prepares the addition of the GX SoC family sound card driver.
The GX sound card, while slightly different, will be similar to the
AXG one. The purpose of this change is to share the utils common to
both sound card driver.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200213155159.3235792-8-jbrunet@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>