cmd: net: add a 'net stats' command to dump network statistics

Add a new option to the 'net' command which can be used to dump network
statistics.

To do this, 3 new callbacks are added to the eth_ops structure:
.get_sset_count(), .get_strings(), .get_stats(). These callbacks
have the same functions as in Linux: to return the number of counters,
the strings which describe those counters and the actual values.

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>