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>