dt-bindings: riscv: add sophgo sg2042 bindings

Add DT binding documentation for the SOPHGO's SG2042 Soc [1] and the
Milk-V Pioneer board [2].

Link: https://en.sophgo.com/product/introduce/sg2042.html [1]
Link: https://milkv.io/pioneer

 [2]

Acked-by: Chao Wei <chao.wei@sophgo.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>