riscv: add ISA extension parsing for Zvfh[min]

Add parsing for Zvfh[min] ISA extension[1] which were ratified in
june 2023 around commit e2ccd0548d6c ("Remove draft warnings from
Zvfh[min]") in riscv-v-spec[2].

Link: https://drive.google.com/file/d/1_Yt60HGAf1r1hx7JnsIptw0sqkBd9BQ8/view [1]
Link: https://github.com/riscv/riscv-v-spec/commits/e2ccd0548d6c

 [2]
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20231114141256.126749-16-cleger@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>