riscv: add ISA extension parsing for bfloat16 ISA extension

riscv: add ISA extension parsing for bfloat16 ISA extension

Add parsing for Zfbmin, Zvfbfmin, Zvfbfwma ISA extension which
were ratified in 4dc23d62 ("Added Chapter title to BF16") of
the riscv-isa-manual.

Signed-off-by: Inochi Amaoto <inochiama@gmail.com>
Reviewed-by: Clément Léger <cleger@rivosinc.com>
Link: https://lore.kernel.org/r/20250213003849.147358-3-inochiama@gmail.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>