arm64: dts: r8a7796: Add external PCIe bus clock

Add the external PCIe bus clock as a zero Hz fixed-frequency clock.
Boards that provide this clock should override it.

Based on r8a7795.dtsi.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/r8a7796.dtsi

@@ -157,6 +157,13 @@
 		clock-frequency = <0>;
 	};
 
+	/* External PCIe clock - can be overridden by the board */
+	pcie_bus_clk: pcie_bus {
+		compatible = "fixed-clock";
+		#clock-cells = <0>;
+		clock-frequency = <0>;
+	};
+
 	soc {
 		compatible = "simple-bus";
 		interrupt-parent = <&gic>;