net: add support for KSZ9477/KSZ9897/KSZ9567 GbE switch

The Microchip KSZ9477/KSZ9897/KSZ9567 7-Port Gigabit Ethernet Switches
support SGMII/RGMII/MII/RMII with register access via SPI, I2C, or MDIO.

This driver currently supports I2C register access but SPI or MDIO register
access can be easily added at a later time.

Tagging is not implemented and instead the active port is tracked to
avoid needing a tag to store port information.

This was tested with the imx8mm-venice-gw7901 board which has a
KSZ9897S switch with an IMX8MM FEC MAC master connected via RGMII_ID.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>