Add description of Zfh[min] ISA extensions[1].

Link: https://drive.google.com/file/d/1z3tQQLm5ALsAD77PM0l0CHnapxWCeVzP/view [1]
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231114141256.126749-12-cleger@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>