net/mlx5: add Rx drop counters to xstats

net/mlx5: add Rx drop counters to xstats

Add two kinds of Rx drop counters to DPDK xstats which are
physical port scope.

1. rx_prio[0-7]_buf_discard
   The number of unicast packets dropped due to lack of shared
   buffer resources.
2. rx_prio[0-7]_cong_discard
   The number of packets that is dropped by the Weighted Random
   Early Detection (WRED) function.

Prio[0-7] is determined by VLAN PCP value which is 0 by default.
Both counters are retrieved from kernel ethtool API which calls
PRM command finally.

Signed-off-by: Rongwei Liu <rongweil@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>