ethtool: add a new command for reading standard stats

Add an interface for reading standard stats, including
stats which don't have a corresponding control interface.

Start with IEEE 802.3 PHY stats. There seems to be only
one stat to expose there.

Define API to not require user space changes when new
stats or groups are added. Groups are based on bitset,
stats have a string set associated.

v1: wrap stats in a nest

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>