Zfhmin is not supported

https://github.com/riscv-software-src/riscv-config/blob/7273a5ed654d0ca9a3b58d4268b19054a9964aa3/riscv_config/constants.py#L28

I get spurious error
```
Input ISA string : RV64IZfhmin does not match accepted canonical ordering
```
because Zfhmin is not supported in this list.

Also it seems that the order is incorrect (correct is `IMAFDC`, but `Zc*` precedes `Zf*`).