dt-bindings: sound: omap-mcpdm: Update documentation for pdmclk

McPDM module receives it's functional clock from external source. This
clock is the pdmclk provided by the twl6040 audio IC. If the clock is not
available all register accesses to McPDM fails and the module is not
operational.

Signed-off-by: Peter Ujfalusi <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/sound/omap-mcpdm.txt

···
        <L3 interconnect address, size>;
 - interrupts: Interrupt number for McPDM
 - ti,hwmods: Name of the hwmod associated to the McPDM
+- clocks:  phandle for the pdmclk provider, likely <&twl6040>
+- clock-names: Must be "pdmclk"
 
 Example:
 
···
 	interrupts = <0 112 0x4>;
 	interrupt-parent = <&gic>;
 	ti,hwmods = "mcpdm";
+};
+
+In board DTS file the pdmclk needs to be added:
+
+&mcpdm {
+	clocks = <&twl6040>;
+	clock-names = "pdmclk";
+	status = "okay";
 };