sxgbe: Add device-tree binding support document

This patch adds binding document for SXGBE ethernet driver via device-tree.

Signed-off-by: Siva Reddy Kallam <siva.kallam@samsung.com>
Signed-off-by: Byungho An <bh74.an@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>