ASoC: meson: axg-fifo: add fifo depth to the bindings documentation

ASoC: meson: axg-fifo: add fifo depth to the bindings documentation

Add a new property with the depth of the fifo in bytes. This is useful
since some instance of the fifo, even on the same SoC, may have different
depth. The depth is useful is set some parameters of the fifo.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20191218172420.1199117-3-jbrunet@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>