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.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/amlogic,axg-fifo.txt

@@ -17,6 +17,9 @@ Required properties:
   * "arb" : memory ARB line (required)
   * "rst" : dedicated device reset line (optional)
 - #sound-dai-cells: must be 0.
+- amlogic,fifo-depth: The size of the controller's fifo in bytes. This
+  		      is useful for determining certain configuration such
+		      as the flush threshold of the fifo
 
 Example of FRDDR A on the A113 SoC:
 
@@ -27,4 +30,5 @@ frddr_a: audio-controller@1c0 {
 	interrupts = <GIC_SPI 88 IRQ_TYPE_EDGE_RISING>;
 	clocks = <&clkc_audio AUD_CLKID_FRDDR_A>;
 	resets = <&arb AXG_ARB_FRDDR_A>;
+	fifo-depth = <512>;
 };