driver: net: ldpaa: Use DPMAC as net device

As per current implementation of DPAA2 ethernet driver DPNI is used as
net device. DPNI is tangible objects can be multiple connected to same physical lane.

Use DPMAC as net device where it represents physical lane.
Below modification done in driver
 - Use global DPNI object
 - Connect DPMAC to DPNI
 - Create and destroy DPMAC

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>