net-next: mediatek: add support for MediaTek MT7622 SoC

This patch adds the driver for ethernet controller on MT7622 SoC. It has
the similar handling logic as the previously MT7623 does, but there are
additions against with MT7623 SoC, the shared SGMII given for the dual
GMACs and including 5-ports 10/100 embedded switch support (ESW) as the
GMAC1 option, thus more clocks consumers for the extra feature are
introduced here. So for ease portability and maintenance, those
differences all are being kept inside the platform data as other drivers
usually do. Currently testing successfully is done with those patches for
the conditions such as GMAC2 with IP1001 PHY via RGMII and GMAC1/2 with
RTL8211F PHY via SGMII.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>