dpaa2-eth: add channel stat to debugfs

Compute the average number of frames processed for each CDAN (Channel
Data Availability Notification) and export it to debugfs detailed
channel stats.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>