riscv: dts: microchip: fix board compatible

According to bindings, the compatible must include microchip,mpfs.  This
fixes dtbs_check warning:

  arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dt.yaml: /: compatible: ['microchip,mpfs-icicle-kit'] is too short
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

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

@@ -10,7 +10,7 @@
 
 / {
 	model = "Microchip PolarFire-SoC Icicle Kit";
-	compatible = "microchip,mpfs-icicle-kit";
+	compatible = "microchip,mpfs-icicle-kit", "microchip,mpfs";
 
 	aliases {
 		ethernet0 = &emac1;

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

@@ -6,8 +6,8 @@
 / {
 	#address-cells = <2>;
 	#size-cells = <2>;
-	model = "Microchip MPFS Icicle Kit";
-	compatible = "microchip,mpfs-icicle-kit";
+	model = "Microchip PolarFire SoC";
+	compatible = "microchip,mpfs";
 
 	chosen {
 	};