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