ftgmac100: Add pause frames configuration and support

Hopefully my understanding of how the hardware works is correct,
as the documentation isn't completely clear. So far I have seen
no obvious issue. Pause seem to also work with NC-SI.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>