stmmac: modified pcs mode support for SGMII

This patch modifies the pcs mode support for SGMII. Even though
SGMII does auto-negotiation with phy, it needs stmmac_init_phy and
stmmac_mdio_register function for initializing phy.

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