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>
diff --git a/drivers/net/dwc_eth_qos_imx.c b/drivers/net/dwc_eth_qos_imx.c
index 962c537..60f3f3f 100644
--- a/drivers/net/dwc_eth_qos_imx.c
+++ b/drivers/net/dwc_eth_qos_imx.c
@@ -55,6 +55,10 @@
 		return -EINVAL;
 	}
 
+	ret = board_interface_eth_init(dev, interface);
+	if (ret)
+		return -EINVAL;
+
 	eqos->max_speed = dev_read_u32_default(dev, "max-speed", 0);
 
 	ret = clk_get_by_name(dev, "stmmaceth", &eqos->clk_master_bus);