drivers: net: add NXP ENETC MDIO driver

Adds a driver for the MDIO interface currently integrated in LS1028A SoC.
This MDIO interface is shared by multiple ethernet interfaces and is
presented as a stand-alone PCI function on the SoC ECAM.
Ethernet has a functional dependency on MDIO, for simplicity there is a
single config option for both.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/drivers/net/Makefile b/drivers/net/Makefile
index a894a42..b6e40df 100644
--- a/drivers/net/Makefile
+++ b/drivers/net/Makefile
@@ -79,4 +79,4 @@
 obj-y += mscc_eswitch/
 obj-$(CONFIG_HIGMACV300_ETH) += higmacv300.o
 obj-$(CONFIG_MDIO_SANDBOX) += mdio_sandbox.o
-obj-$(CONFIG_FSL_ENETC) += fsl_enetc.o
+obj-$(CONFIG_FSL_ENETC) += fsl_enetc.o fsl_enetc_mdio.o