dt-bindings: riscv: microchip: Add YAML documentation for the PolarFire SoC

Add YAML DT binding documentation for the Microchip PolarFire SoC.
It is documented at:

https://www.microsemi.com/products/fpga-soc/polarfire-soc-icicle-quick-start-guide



Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>