dt-bindings: net: cdns,macb: add sam9x7 ethernet interface

Add documentation for sam9x7 ethernet interface.

Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240223172228.671553-1-varshini.rajendran@microchip.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>