net: Introduce devicetree aware DEVICE define macros

Provide versions of NET_DEVICE_INIT, NET_DEVICE_INIT_INSTANCE, and
NET_DEVICE_OFFLOAD_INIT that are both devicetree node and instance
aware.  We use the _DEFINE suffix for the DT versions to match the
naming convention in device.h

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>