dt-bindings: net: Add support for Sophgo CV1800 dwmac

The GMAC IP on CV1800 series SoC is a standard Synopsys
DesignWare MAC (version 3.70a).

Add necessary compatible string for this device.

Signed-off-by: Inochi Amaoto <inochiama@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20250703021220.124195-1-inochiama@gmail.com


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