powerpc/sgmii: To support PHY link state auto detect in SGMII mode

PHYs on SGMII riser card are used in SGMII mode with different external
IRQs from eTSEC. This means in SGMII mode phy-handle and phy-connection-type
under ethernet node should be updated. Otherwise the PHY interrupt can not
be handled therefor PHY link state change can not be auto detected.

For we have seperate SGMII PHY nodes, ethernet PHY reg fixup is not needed
but it's still be kept to guarantee the sgmii mode could work with old
device tree.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Jia Hongtao <B38951@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>