net: stmmac: add ethtool per-queue statistic framework

Adding generic ethtool per-queue statistic framework to display the
statistics for each rx/tx queue. In future, users can avail it to add
more per-queue specific counters. Number of rx/tx queues displayed is
depending on the available rx/tx queues in that particular MAC config
and this number is limited up to the MTL_MAX_{RX|TX}_QUEUES defined
in the driver.

Ethtool per-queue statistic display will look like below, when users
start adding more counters.

Example:
 q0_tx_statA:
 q0_tx_statB:
 q0_tx_statC:
 |
 q0_tx_statX:
 .
 .
 .
 qMAX_tx_statA:
 qMAX_tx_statB:
 qMAX_tx_statC:
 |
 qMAX_tx_statX:

 q0_rx_statA:
 q0_rx_statB:
 q0_rx_statC:
 |
 q0_rx_statX:
 .
 .
 .
 qMAX_rx_statA:
 qMAX_rx_statB:
 qMAX_rx_statC:
 |
 qMAX_rx_statX:

In addition, this patch has the support on displaying the number of
packets received and transmitted per queue.

Signed-off-by: Vijayakannan Ayyathurai <vijayakannan.ayyathurai@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>