arm: ls1021atwr: Convert to use driver model TSEC driver

Now that we have added driver model support to the TSEC driver,
convert ls1021atwr board to use it.

This depends on previous DM series for ls1021atwr:
http://patchwork.ozlabs.org/patch/561855/



Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

[Vladimir] Made the following changes:
- Added 'status = "disabled";' for all Ethernet ports in ls1021a.dtsi
- Fixed the confusion between the SGMII/TBI PCS for enet0 and enet1 -
  a mistake ported over from Linux. Each SGMII PCS lies on the private
  MDIO bus of the interface (and the RGMII enet2 has no SGMII PCS).
- Added CONFIG_DM_ETH to all ls1021atwr_* defconfigs
- Completely removed non-DM_ETH support from ls1021atwr
- Changed "compatible" string from "fsl,tsec-mdio" to "fsl,etsec2-mdio"
  and from "fsl,tsec" to "fsl,etsec2" to match Linux