riscv: dts: sifive: add missing compatible for plic

Add proper compatible for Platform-Level Interrupt Controller to silence
dtbs_check warnings:

  interrupt-controller@c000000: compatible: ['sifive,plic-1.0.0'] is too short
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Tested-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Link: https://lore.kernel.org/r/20210920130412.145231-2-krzysztof.kozlowski@canonical.com

arch/riscv/boot/dts/sifive/fu540-c000.dtsi

@@ -141,7 +141,7 @@ soc {
 		ranges;
 		plic0: interrupt-controller@c000000 {
 			#interrupt-cells = <1>;
-			compatible = "sifive,plic-1.0.0";
+			compatible = "sifive,fu540-c000-plic", "sifive,plic-1.0.0";
 			reg = <0x0 0xc000000 0x0 0x4000000>;
 			riscv,ndev = <53>;
 			interrupt-controller;