net: ethernet: mtk-star-emac: new driver

This adds the driver for the MediaTek STAR Ethernet MAC currently used
on the MT8* SoC family. For now we only support full-duplex.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>