riscv: dts: sifive: fu540-c000: Drop bogus soc node compatible values

"make dtbs_check":

    arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dt.yaml: soc: $nodename:0: '/' was expected
    	From schema: Documentation/devicetree/bindings/riscv/sifive.yaml
    arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dt.yaml: soc: compatible: 'oneOf' conditional failed, one must be fixed:
    	'sifive,fu540-c000' is not one of ['sifive,hifive-unleashed-a00']
    	'sifive,fu540-c000' is not one of ['sifive,hifive-unmatched-a00']
    	'sifive,fu540-c000' was expected
    	'sifive,fu740-c000' was expected
    	'sifive,fu540' was expected
    	'sifive,fu740' was expected
    	From schema: Documentation/devicetree/bindings/riscv/sifive.yaml

This happens because the "soc" subnode declares compatibility with
"sifive,fu540-c000" and "sifive,fu540", while these are only intended
for the root node.

Fix this by removing the bogus compatible values from the "soc" node.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

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

@@ -137,7 +137,7 @@
 	soc {
 		#address-cells = <2>;
 		#size-cells = <2>;
-		compatible = "sifive,fu540-c000", "sifive,fu540", "simple-bus";
+		compatible = "simple-bus";
 		ranges;
 		plic0: interrupt-controller@c000000 {
 			#interrupt-cells = <1>;