net: fec: sharing MDIO for two enet controllers

On i.MX6SX, 6UL and 7D, there are two enet controllers each has a
MDIO port. But Some boards share one 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.

To i.MX28, adapt to use the new config

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>