Add support for pause specific stats.

Tested on ksz9477.
Signed-off-by: NOleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: NVladimir Oltean <olteanv@gmail.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>