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

Add documentation for SAMA5D29 ethernet interface.
Signed-off-by: NHari Prasath <Hari.PrasathGE@microchip.com>
Link: https://lore.kernel.org/r/20210812074422.13487-2-Hari.PrasathGE@microchip.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/macb.txt

@@ -8,6 +8,7 @@ Required properties:
   Use "cdns,np4-macb" for NP4 SoC devices.
   Use "cdns,at32ap7000-macb" for other 10/100 usage or use the generic form: "cdns,macb".
   Use "atmel,sama5d2-gem" for the GEM IP (10/100) available on Atmel sama5d2 SoCs.
+  Use "atmel,sama5d29-gem" for GEM XL IP (10/100) available on Atmel sama5d29 SoCs.
   Use "atmel,sama5d3-macb" for the 10/100Mbit IP available on Atmel sama5d3 SoCs.
   Use "atmel,sama5d3-gem" for the Gigabit IP available on Atmel sama5d3 SoCs.
   Use "atmel,sama5d4-gem" for the GEM IP (10/100) available on Atmel sama5d4 SoCs.