net: dpaa2-eth: add support for multiple buffer pools per DPNI

This patch allows the configuration of multiple buffer pools associated
with a single DPNI object, each distinct DPBP object not necessarily
shared among all queues.
The user can interogate both the number of buffer pools and the buffer
count in each buffer pool by using the .get_ethtool_stats() callback.

Signed-off-by: Robert-Ionut Alexa <robert-ionut.alexa@nxp.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>