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>

Author: clementleger

Documentation/devicetree/bindings/riscv/extensions.yaml

@@ -214,6 +214,12 @@ properties:
             instructions as ratified at commit 6d33919 ("Merge pull request #158
             from hirooih/clmul-fix-loop-end-condition") of riscv-bitmanip.
 
+        - const: zfa
+          description:
+            The standard Zfa extension for additional floating point
+            instructions, as ratified in commit 056b6ff ("Zfa is ratified") of
+            riscv-isa-manual.
+
         - const: zfh
           description:
             The standard Zfh extension for 16-bit half-precision binary