riscv: add ISA extension parsing for Zfa

Add parsing for Zfa ISA extension [1] which were ratified in commit
056b6ff467c7 ("Zfa is ratified") of riscv-isa-manual[2].

Link: https://drive.google.com/file/d/1VT6QIggpb59-8QRV266dEE4T8FZTxGq4/view [1]
Link: https://github.com/riscv/riscv-isa-manual/commits/056b6ff467c7

 [2]
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20231114141256.126749-19-cleger@rivosinc.com


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