ASoC: meson: axg-fifo: improve depth handling

ASoC: meson: axg-fifo: improve depth handling

Let the fifo driver parse the fifo depth from DT. Eventually all DT should
have this property. Until it is actually the case, default to 256 bytes if
the property is missing. 256 bytes is the size of the smallest fifo on the
supported SoCs.

On the supported SoC, fifo A is usually bigger than the other ones.  With
depth known, we can improve the usage of the fifo and adapt the setup of
request threshold.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20191218172420.1199117-4-jbrunet@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>