dt-bindings: arm: at91: Document lan966 pcb8291 and pcb8290 boards

Add documentation for Microchip LAN9662 PCB8291 and
LAN9668 PCB8290 Evaluation Boards.
Signed-off-by: Kavyasree Kotagiri <kavyasree.kotagiri@microchip.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20211004105926.5696-4-kavyasree.kotagiri@microchip.com

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

@@ -150,6 +150,18 @@ properties:
           - const: microchip,sama7g5
           - const: microchip,sama7
 
+      - description: Microchip LAN9662 PCB8291 Evaluation Board.
+        items:
+          - const: microchip,lan9662-pcb8291
+          - const: microchip,lan9662
+          - const: microchip,lan966
+
+      - description: Microchip LAN9668 PCB8290 Evaluation Board.
+        items:
+          - const: microchip,lan9668-pcb8290
+          - const: microchip,lan9668
+          - const: microchip,lan966
+
       - items:
           - enum:
               - atmel,sams70j19