dt-bindings: vendor-prefixes: add milkv/sophgo

Add new vendor strings to dt bindings.
These new vendor strings are used by
- SOPHGO's SG2042 SoC [1]
- Milk-V Pioneer board [2], which uses SG2042 chip.

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

 [2]

Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
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>