dt-bindings: riscv: add Zacas ISA extension description

Add description for the Zacas ISA extension which was ratified recently.
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231220155723.684081-4-cleger@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/riscv/extensions.yaml

@@ -171,6 +171,12 @@ properties:
             memory types as ratified in the 20191213 version of the privileged
             ISA specification.
 
+        - const: zacas
+          description: |
+            The Zacas extension for Atomic Compare-and-Swap (CAS) instructions
+            is supported as ratified at commit 5059e0ca641c ("update to
+            ratified") of the riscv-zacas.
+
         - const: zba
           description: |
             The standard Zba bit-manipulation extension for address generation