ASoC: rsnd: audio_clkout0/1/2/3 are optional properties

Renesas sound driver can output Audio-clkout0/1/2/3,
but these are optional properties for each board.
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/renesas,rsnd.txt

@@ -34,6 +34,8 @@ Required properties:
 				  see below for detail.
 - #sound-dai-cells		: it must be 0 if your system is using single DAI
 				  it must be 1 if your system is using multi  DAI
+
+Optional properties:
 - #clock-cells			: it must be 0 if your system has audio_clkout
 				  it must be 1 if your system has audio_clkout0/1/2/3
 - clock-frequency		: for all audio_clkout0/1/2/3