dt-bindings: net: macb: add documentation for sama5d29 ethernet interface

Add documentation for SAMA5D29 ethernet interface.

Signed-off-by: Hari Prasath <Hari.PrasathGE@microchip.com>
Link: https://lore.kernel.org/r/20210812074422.13487-2-Hari.PrasathGE@microchip.com


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