ARM: dts: r8a7793: add audio clock to device tree

Instantiate audio clock in r8a7793 device tree.
audio_clk_a/b/c are required for R-Car sound.

Based on similar work for the r8a7791 by Kuninori Morimoto.

Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/boot/dts/r8a7793.dtsi

@@ -706,6 +706,29 @@ extal_clk: extal_clk {
 			clock-output-names = "extal";
 		};
 
+		/*
+		 * The external audio clocks are configured as 0 Hz fixed frequency clocks by
+		 * default. Boards that provide audio clocks should override them.
+		 */
+		audio_clk_a: audio_clk_a {
+			compatible = "fixed-clock";
+			#clock-cells = <0>;
+			clock-frequency = <0>;
+			clock-output-names = "audio_clk_a";
+		};
+		audio_clk_b: audio_clk_b {
+			compatible = "fixed-clock";
+			#clock-cells = <0>;
+			clock-frequency = <0>;
+			clock-output-names = "audio_clk_b";
+		};
+		audio_clk_c: audio_clk_c {
+			compatible = "fixed-clock";
+			#clock-cells = <0>;
+			clock-frequency = <0>;
+			clock-output-names = "audio_clk_c";
+		};
+
 		/* Special CPG clocks */
 		cpg_clocks: cpg_clocks@e6150000 {
 			compatible = "renesas,r8a7793-cpg-clocks",