ARM: dts: r8a7778: Add missing clock-frequency for fixed clocks

"clock-frequency" is a required property for devices nodes compatible
with "fixed-clock", leading to warnings when running

    $ make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/clock/fixed-clock.yaml
    arch/arm/boot/dts/r8a7778-bockw.dt.yaml: audio_clk_a: 'clock-frequency' is a required property
    arch/arm/boot/dts/r8a7778-bockw.dt.yaml: audio_clk_b: 'clock-frequency' is a required property
    arch/arm/boot/dts/r8a7778-bockw.dt.yaml: audio_clk_c: 'clock-frequency' is a required property

Fix this by adding the missing "clock-frequency" properties to the audio
clocks, to be overridden by board DTS files when populated.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191213162712.2056-1-geert+renesas@glider.be

arch/arm/boot/dts/r8a7778.dtsi

@@ -498,14 +498,17 @@
 		audio_clk_a: audio_clk_a {
 			compatible = "fixed-clock";
 			#clock-cells = <0>;
+			clock-frequency = <0>;
 		};
 		audio_clk_b: audio_clk_b {
 			compatible = "fixed-clock";
 			#clock-cells = <0>;
+			clock-frequency = <0>;
 		};
 		audio_clk_c: audio_clk_c {
 			compatible = "fixed-clock";
 			#clock-cells = <0>;
+			clock-frequency = <0>;
 		};
 
 		/* Fixed ratio clocks */