ASoC: wm8962: Add optional mclk device tree binding

The driver can request an optional clock for mclk.
Update the txt file to reflect this.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201217162740.1452000-1-aford173@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/wm8962.txt

@@ -9,6 +9,9 @@ Required properties:
   - reg : the I2C address of the device.
 
 Optional properties:
+
+  - clocks : The clock source of the mclk
+
   - spk-mono: This is a boolean property. If present, the SPK_MONO bit
     of R51 (Class D Control 2) gets set, indicating that the speaker is
     in mono mode.
@@ -27,6 +30,7 @@ Example:
 wm8962: codec@1a {
 	compatible = "wlf,wm8962";
 	reg = <0x1a>;
+	clocks = <&clks IMX6QDL_CLK_CKO>;
 
 	gpio-cfg = <
 		0x0000 /* 0:Default */