net: mediatek: add support for MediaTek MT7988 SoC

This patch adds support for MediaTek MT7988.

MT7988 features MediaTek NETSYS v3, including three GMACs, and two
of them supports 10Gbps USXGMII.

MT7988 embeds a MT7531 switch (not MCM) which supports accessing
internal registers through MMIO instead of MDIO.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>