On i.MX6SX, 6UL and 7D, there are two enet controllers each has a
MDIO port. Some boards share the MDIO port for the two enets. So
introduce a configuration CONFIG_FEC_MXC_MDIO_BASE to indicate
the MDIO port for sharing.
In Kconfig, user needs enable CONFIG_FEC_MXC_SHARE_MDIO first to enter
the CONFIG_FEC_MXC_MDIO_BASE.

Without defining this configuration, the enet will uses own MDIO port.
Also add this configurationfor FEC DM driver, since the ENET PHY currently
does not support FDT.

Modify the dev_id for DM driver, the first fec will set dev_id with 0, while
second fec sets the dev_id with 1. So the MII bus name won't be duplicated.
Then we can add two FEC devices from DTB.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 088afc073c480bd02208145bfb04259ccdf391c1)