dt-bindings: net: renesas,rzn1-gmac: Document RZ/N1 GMAC support

dt-bindings: net: renesas,rzn1-gmac: Document RZ/N1 GMAC support

The RZ/N1 series of MPUs feature up to two Gigabit Ethernet controllers.
These controllers are based on Synopsys IPs. They can be connected to
RZ/N1 RGMII/RMII converters.

Add a binding that describes these GMAC devices.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
[rgantois: commit log]
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Romain Gantois <romain.gantois@bootlin.com>
Link: https://lore.kernel.org/r/20240513-rzn1-gmac1-v7-1-6acf58b5440d@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>