riscv: dts: microchip: use an mpfs specific l2 compatible

PolarFire SoC does not have the same l2 cache controller as the fu540,
featuring an extra interrupt. Appease the devicetree checker overlords
by adding a PolarFire SoC specific compatible to fix the below sort of
warnings:

mpfs-polarberry.dtb: cache-controller@2010000: interrupts: [[1], [3], [4], [2]] is too long

Fixes: 0fa6107e ("RISC-V: Initial DTS for Microchip ICICLE board")
Fixes: 34fc9cc3 ("riscv: dts: microchip: correct L2 cache interrupts")
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

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

@@ -185,7 +185,7 @@ soc {
 		ranges;
 
 		cctrllr: cache-controller@2010000 {
-			compatible = "sifive,fu540-c000-ccache", "cache";
+			compatible = "microchip,mpfs-ccache", "sifive,fu540-c000-ccache", "cache";
 			reg = <0x0 0x2010000 0x0 0x1000>;
 			cache-block-size = <64>;
 			cache-level = <2>;