net: dsa: bcm_sf2: Add support for CFP statistics

Return CFP policer statistics (Green, Yellow or Red) as part of the
standard ethtool statistics. This helps debug when CFP rules may not be
hit (0 counter).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>