dt-bindings: riscv: microchip: document the Aldec TySoM

The TySOM-M-MPFS250 is a compact SoC prototyping board featuring
a Microchip PolarFire SoC MPFS250T-FCG1152.
Document rev2 of this board.

Link: https://www.aldec.com/en/products/emulation/tysom_boards/polarfire_microchip/tysom_m_mpfs250


Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>