ARM: dts: imx6ul: segin: Make FEC and ethphy configurable in dts

To disable Ethernet interfaces in case they are not populated
make the FEC and Ethernet PHY status configurable in the dts files.

Also change the Ethernet PHYs labels to make them correspond to
the MDIO address.

Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>