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>