dt-bindings: riscv: Add Andes PMU extension description

Document the ISA string for Andes Technology performance monitor
extension which provides counter overflow interrupt and mode
filtering mechanisms.
Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20240222083946.3977135-9-peterlin@andestech.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/riscv/extensions.yaml

@@ -477,5 +477,12 @@ properties:
             latency, as ratified in commit 56ed795 ("Update
             riscv-crypto-spec-vector.adoc") of riscv-crypto.
 
+        - const: xandespmu
+          description:
+            The Andes Technology performance monitor extension for counter overflow
+            and privilege mode filtering. For more details, see Counter Related
+            Registers in the AX45MP datasheet.
+            https://www.andestech.com/wp-content/uploads/AX45MP-1C-Rev.-5.0.0-Datasheet.pdf
+
 additionalProperties: true
 ...