dts: intel_socfpga: bugfix, add emac-index in dts

Bugfix for the Cyclone V SoC DK ethernet driver need to add emac-index
in dts
 - We remove the "local-mac-address" property from
   dts/arm/intel_socfpga_std.dtsi to
   boards/arm/cyclonev_socdk/cyclonev_socdk.dts, since this value is
   dependant on the board / implementation and not universal to
   the "intel_socfpga" package that it inherets from.
 - The above is also true for the "status" property as the board
   should enable the device.

Signed-off-by: Benjamin Kyd <benjamin.kyd@intel.com>