net: phy: broadcom: Add support code for reading PHY counters

Broadcom PHYs expose a number of PHY error counters: receive errors,
false carrier sense, SerDes BER count, local and remote receive errors.
Add support code to allow retrieving these error counters. Since the
Broadcom PHY library code is used by several drivers, make it possible
for them to specify the storage for the software copy of the statistics.

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