dpaa2-eth: extract the S/G table buffer cache interaction into functions

The dpaa2-eth driver uses in certain circumstances a buffer cache for
the S/G tables needed in case of a S/G FD. At the moment, the
interraction with the cache is open-coded and couldn't be reused easily.

Add two new functions - dpaa2_eth_sgt_get and dpaa2_eth_sgt_recycle -
which help with code reusability.

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