net: dwc_eth_qos: Add board_interface_eth_init() for i.MX8M Plus

Implement common board_interface_eth_init() and call it from the DWMAC
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 Plus DWMAC and supersedes current board-side configuration
of the same IOMUX GPR[1] duplicated in the board files.

Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>