net: thunderx: Pause frame support

Enable pause frames on both Rx and Tx side, configure pause
interval e.t.c. Also support for enable/disable pause frames
on Rx/Tx via ethtool has been added.

Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>