drivers: net: xgene-v2: Extend ethtool statistics

This patch adds extended statistics reporting to ethtool.

In summary, this patch,

   - adds ethtool.h with the statistics register definitions
   - adds 'struct xge_gstrings_extd_stats' to gather extended stats
   - modifies xge_get_strings(), get_sset_count() and
     get_ethtool_stats() accordingly
   - moves 'struct xge_gstrings_stats' to ethtool.h

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>