riscv: dts: sophgo: add Milk-V Pioneer board device tree

riscv: dts: sophgo: add Milk-V Pioneer board device tree

Milk-V Pioneer [1] is a developer motherboard based on SG2042
in a standard mATX form factor.

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

Link: https://milkv.io/pioneer
Reviewed-by: Guo Ren <guoren@kernel.org>
Acked-by: Chao Wei <chao.wei@sophgo.com>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>