dt-bindings: riscv: cpus: reg matches hart ID

Add a description to the CPU reg property to clarify that
the reg property must match the hart ID.
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -75,6 +75,10 @@ properties:
       - riscv,sv57
       - riscv,none
 
+  reg:
+    description:
+      The hart ID of this CPU node.
+
   riscv,cbom-block-size:
     $ref: /schemas/types.yaml#/definitions/uint32
     description: