dt-bindings: ARM: at91: make separate entry for Olimex board

The Olimex board exists with only one SoC option. Change the bindings to
reflect that.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250226084938.3436-7-wsa+renesas@sang-engineering.com
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>

Author: Wolfram Sang

Documentation/devicetree/bindings/arm/atmel-at91.yaml

@@ -22,8 +22,6 @@ properties:
       - items:
           - const: atmel,at91rm9200
       - items:
-          - enum:
-              - olimex,sam9-l9260
           - enum:
               - atmel,at91sam9260
               - atmel,at91sam9261
@@ -36,6 +34,12 @@ properties:
               - atmel,at91sam9x60
           - const: atmel,at91sam9
 
+      - description: Olimex SAM9-L9260
+        items:
+          - const: olimex,sam9-l9260
+          - const: atmel,at91sam9260
+          - const: atmel,at91sam9
+
       - items:
           - enum:
               - overkiz,kizboxmini-base # Overkiz kizbox Mini Base Board