dpaa2-eth: add software counter for Tx frames converted to S/G

With the previous commit, in case of insufficient SKB headroom on the Tx
path instead of reallocing the SKB we now send a S/G frame descriptor.
Export the number of occurences of this case as a per CPU counter (in
debugfs) and a total number in the ethtool statistics - "tx converted sg
frames'.

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