dt-bindings: riscv: starfive: add Milkv Mars board

Add device tree bindings for the Milkv Mars board which is
equipped with StarFive JH7110 SoC.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>