dts: vendor-prefixes: add starfive

We have various compatibles matching this in a DTSI file.

Linux tracks this vendor too:

https://github.com/torvalds/linux/commit/4071883fd8f386fa070cf82b30909ea5ddd7dcf9



Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>