dt-bindings: display: sii902x: Change audio mclk binding

As stated in SiL9022/24 datasheet, master clock is not required for I2S.
Make mclk property optional in DT bindings.

Fixes: 3f18021f43a3 ("dt-bindings: display: sii902x: Add HDMI audio bindings")

Signed-off-by: Olivier Moysan <[e-mail]>
Reviewed-by: Jyri Sarha <[e-mail]>
Acked-by: Andrzej Hajda <[e-mail]>
Signed-off-by: Benjamin Gaignard <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/devicetree/bindings/display/bridge/sii902x.txt

···
 	- clocks: phandle and clock specifier for each clock listed in
            the clock-names property
 	- clock-names: "mclk"
-	   Describes SII902x MCLK input. MCLK is used to produce
-	   HDMI audio CTS values. This property is required if
-	   "#sound-dai-cells"-property is present. This property follows
+	   Describes SII902x MCLK input. MCLK can be used to produce
+	   HDMI audio CTS values. This property follows
 	   Documentation/devicetree/bindings/clock/clock-bindings.txt
 	   consumer binding.