dt-bindings: net: fsl: enetc: Add bindings for the central MDIO PCIe endpoint

The on-chip PCIe root complex that integrates the ENETC ethernet
controllers also integrates a PCIe endpoint for the MDIO controller
providing for centralized control of the ENETC mdio bus.
Add bindings for this "central" MDIO Integrated PCIe Endpoint.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>