dsa: mv88e6xxx: Add Second back of statistics

The 6320 family of switch chips has a second bank for statistics, but
is missing three statistics in the port registers. Generalise and
extend the code:

* adding a field to the statistics table indicating the bank/register
  set where each statistics is.
* add a function indicating if an individual statistics
  is available on this device
* calculate at run time the sset_count.
* return strings based on the available statistics of the device
* return statistics based on the available statistics of the device
* Add support for reading from the second bank.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>