dt-bindings: riscv: microchip: document icicle reference design

Add a compatible for the icicle kit's reference design. This represents
the FPGA fabric's contents & is versioned to denote which release of the
reference design it applies to.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220509142610.128590-5-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>