net: dpaa2-eth: export buffer pool info into a new debugfs file

Export the allocated buffer pools, the number of buffers that they have
currently and which channels are using which BP.

The output looks like below:

Buffer pool info for eth2:
IDX        BPID      Buf count      CH#0      CH#1      CH#2      CH#3
BP#0         1           5124         x         x         x         x

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