net: designware: Fix for use with current Linux device tree for Meson GX

In Uboot for Meson GX the compatible string in meson-gxbb.dtsi so far is:
compatible = "amlogic,meson6-dwmac", "snps,dwmac";

On Linux in the same dt file it's
compatible = "amlogic,meson-gx-dwmac", "amlogic,meson-gxbb-dwmac", "snps,dwmac";

To avoid breaking ethernet with the next DT synch from Linux to U-Boot
(planned as prerequisite for adding Meson GX MMC driver to U-Boot) add
"amlogic,meson-gx-dwmac" to the compatibility list in the designware
driver.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>