net: designware: add Amlogic Meson8b & later glue driver

This adds a proper glue driver for the Designware DWMAC ethernet MAC IP
found in the Amlogic Meson8, GXBB, GXL, GXM, G12A, G12B & SM1 SoCs.

This is aimed to replace the static ethernet link setup found on the board
init code for the Amlogic SoC based boards.

Tested on a libretech-cc (S905x Internal RMII 10/100 PHY) and Khadas VIM3 (A113d
with external 10/100/1000 RGMII PHY) to cover the most extreme setups.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>