dt-bindings: arm: at91: add lan966 pcb8309 board

Add documentation for Microchip LAN9662 PCB8309.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220722131836.2377720-2-horatiu.vultur@microchip.com