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>