U-Boot for Loongson Boards

Collect open source port of U-Boot to Loongson Boards by Loongson:

1. Loongson port based on U-Boot v2022.04 in branch loongson-2022.04 found via:
   1. Access https://github.com/loongarch64/projX-2k500-kernel, find the baiduyun download link.
   2. Locate the tarball at 广东龙芯2K500-先锋板-v1.0/02-固件/源码/u-boot-2022.04-opensrc-40cb375-build.20231207201231.tar.gz

An alternative version can be found via: Access https://github.com/LoongsonLab/2k1000-materials, find the baiduyun download link.