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>