net/ucc_geth: Add SGMII support for UCC GETH driver

-- derived from reverted commit 047584ce
-- reworked by Grant Likely to play nice with commit:
   "net: Rework ucc_geth driver to use of_mdio infrastructure"
   (0b9da337)

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>