dt-bindings: riscv: sifive-l2: add a PolarFire SoC compatible

The l2 cache on PolarFire SoC is cross between that of the fu540 and
the fu740. It has the extra interrupt from the fu740 but the lower
number of cache-sets. Add a specific compatible to avoid the likes
of:

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

Fixes: 34fc9cc3

 ("riscv: dts: microchip: correct L2 cache interrupts")
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>