net/mlx5e: Statistics handling refactoring

Redesign ethtool statistics handling and reporting in the driver:
1. Move counters to a separate file (en_stats.h).
2. Remove unnecessary dependencies between stats and strings.
3. Use counter descriptors which hold a name and offset for each counter,
   and will be used to decide which counters will be exposed.

For example when adding a new software counter to ethtool, instead of:
1. Add to stats struct.
2. Add to strings struct in the same order.
3. Change macro defining number of software counters.
The only thing needed is to link the new counter to a counter descriptor.

VPort counters are a set of hardware traffic counters created automatically
for each virtual port opened.
PPort counters are a set of counters describing per physical port
performance statistics.
These counters are gathered from hardware register and divided to groups
according to different protocols.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>