net: ldpaa_eth: export DPNI and DPMAC counters through 'net stats'

Export the already existing DPNI and DPMAC counters through the newly
added callbacks.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>