net: ldpaa_eth: extend debug capabilities with DPMAC statistics

The ldpaa_eth driver already had a DPMAC statistics dump, this patch
extends the list of stats and adds a bit more structure to the code.

For a bit more context, the DPAA2 u-boot software architecture uses a
default network interface object - a DPNI - which, at runtime, will get
connected to the currently used DPMAC object.
Each time the .stop() eth callback is called, the DPMAC is destroyed
thus any previous counters will get lost.

As a preparation for the next patches, we add a software kept set of
DPMAC counters which will get updated before each destroy operation
takes place.

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>