drivers: net: fsl_enetc: register internal MDIO bus

This bus is used to access internal SoC PHYs.  These PHYs are configured
by the ENETC driver directly, but it's useful to have command line access
to this MDIO to debug the system especially when using new external PHYs.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>