riscv: dts: microchip: mpfs: Fix reference clock node

"make dtbs_check" reports:

    arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dt.yaml: soc: refclk: {'compatible': ['fixed-clock'], '#clock-cells': [[0]], 'clock-frequency': [[600000000]], 'clock-output-names': ['msspllclk'], 'phandle': [[7]]} should not be valid under {'type': 'object'}
	From schema: dtschema/schemas/simple-bus.yaml

Fix this by moving the node out of the "soc" subnode.
While at it, rename it to "msspllclk", and drop the now superfluous
"clock-output-names" property.
Move the actual clock-frequency value to the board DTS, since it is not
set until bitstream programming time.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dts

@@ -35,6 +35,10 @@
 	};
 };
 
+&refclk {
+	clock-frequency = <600000000>;
+};
+
 &serial0 {
 	status = "okay";
 };

arch/riscv/boot/dts/microchip/microchip-mpfs.dtsi

@@ -139,6 +139,11 @@
 		};
 	};
 
+	refclk: msspllclk {
+		compatible = "fixed-clock";
+		#clock-cells = <0>;
+	};
+
 	soc {
 		#address-cells = <2>;
 		#size-cells = <2>;
@@ -189,13 +194,6 @@
 			#dma-cells = <1>;
 		};
 
-		refclk: refclk {
-			compatible = "fixed-clock";
-			#clock-cells = <0>;
-			clock-frequency = <600000000>;
-			clock-output-names = "msspllclk";
-		};
-
 		clkcfg: clkcfg@20002000 {
 			compatible = "microchip,mpfs-clkcfg";
 			reg = <0x0 0x20002000 0x0 0x1000>;