drivers: ethernet: phy: add Microchip's KSZ9131 PHY support Add support for KSZ9131 (Gigabit Ethernet Transceiver with RGMII Support). As first starter, 100MBit/s mode is tested. https://www.microchip.com/en-us/product/ksz9131 Signed-off-by: Tony Han <tony.han@microchip.com>