dt-bindings: net: Add T-HEAD dwmac support

dt-bindings: net: Add T-HEAD dwmac support

Add documentation to describe the DesginWare-based GMAC controllers in
the T-HEAD TH1520 SoC.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>
Link: https://patch.msgid.link/20241103-th1520-gmac-v7-1-ef094a30169c@tenstorrent.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>