dt-bindings: riscv: add Zfa ISA extension description

Add description for the Zfa ISA extension[1].

Link: https://drive.google.com/file/d/1VT6QIggpb59-8QRV266dEE4T8FZTxGq4/view

 [1]
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231114141256.126749-21-cleger@rivosinc.com


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