Add Sipeed as Vendor

[wucke13]

Sipeed offers quite a few interesting very low cost RISC V boards, like the Sipeed Longan Nano.
The cpu - a GD32 - is officially supported by the RISC V Team, in this crate: https://github.com/riscv-rust/gd32vf103xx-hal

Adding Sipeed and a link to that Repo would be nice, so that people don't think there board isn't supported after they looked hear.

[Disasm]

PR would be great!