dt-bindings: riscv: Add smstateen entry

Add an entry for the Smstateen extension to the riscv,isa-extensions
property.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Anup Patel <anup@brainfault.org>

Author: mdchitale

Documentation/devicetree/bindings/riscv/extensions.yaml

@@ -128,6 +128,12 @@ properties:
             changes to interrupts as frozen at commit ccbddab ("Merge pull
             request #42 from riscv/jhauser-2023-RC4") of riscv-aia.
 
+        - const: smstateen
+          description: |
+            The standard Smstateen extension for controlling access to CSRs
+            added by other RISC-V extensions in H/S/VS/U/VU modes and as
+            ratified at commit a28bfae (Ratified (#7)) of riscv-state-enable.
+
         - const: ssaia
           description: |
             The standard Ssaia supervisor-level extension for the advanced