net: fec_mxc: Add board_interface_eth_init() for i.MX8M Mini/Nano/Plus

Implement common board_interface_eth_init() and call it from the FEC
driver to configure IOMUXC GPR[1] register according to the PHY mode
obtained from DT. This supports all three interface modes supported by
the i.MX8M Mini/Nano/Plus FEC and supersedes the current board-side
configuration of the same IOMUX GPR[1] duplicated in the board files.

Signed-off-by: Marek Vasut <marex@denx.de>