riscv: dts: add initial Sophgo SG2042 SoC device tree

Milk-V Pioneer motherboard is powered by SG2042.

SG2042 is server grade chip with high performance, low power
consumption and high data throughput.
Key features:
- 64 RISC-V cpu cores
- 4 cores per cluster, 16 clusters on chip
- More info is available at [1].

Currently only support booting into console with only uart,
other features will be added soon later.

Link: https://en.sophgo.com/product/introduce/sg2042.html

 [1]
Reviewed-by: Guo Ren <guoren@kernel.org>
Acked-by: Chao Wei <chao.wei@sophgo.com>
Co-developed-by: Xiaoguang Xing <xiaoguang.xing@sophgo.com>
Signed-off-by: Xiaoguang Xing <xiaoguang.xing@sophgo.com>
Co-developed-by: Inochi Amaoto <inochiama@outlook.com>
Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>