Maintainers.txt: Add maintainers for Sophgo platform

This "Platform/Sophgo/Maintainers.md" file format is useless with
edk2's "BaseTools/Scripts/GetMaintainer.py" utility, so delete this
file. Add add the maintainers to the "Maintainers.txt" file.

Cc: dahogn <dahogn@hotmail.com>
Cc: meng-cz <mengcz1126@gmail.com>
Cc: USER0FISH <libing1202@outlook.com>
Cc: Sunil V L <sunilvl@ventanamicro.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: caiyuqing379 <caiyuqing_hz@outlook.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>