dpaa2-eth: Add QBMAN related stats

Add statistics for pending frames in Rx/Tx conf FQs and
number of buffers in pool. Available through ethtool -S.

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