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: 3f18021f ("dt-bindings: display: sii902x: Add HDMI audio bindings")
Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1563811560-29589-3-git-send-email-olivier.moysan@st.com

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

@@ -26,9 +26,8 @@ Optional properties:
 	- 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.