stmmac: intel: prepare to support 1000BASE-X phy interface setting

Currently, intel_speed_mode_2500() redundantly fix-up phy_interface to
PHY_INTERFACE_MODE_SGMII if the underlying controller is in 1000Mbps
SGMII mode. The value of phy_interface has been initialized earlier.

This patch removes such redundancy to prepare for setting 1000BASE-X
mode for certain hardware platform configuration.

Also update the intel_mgbe_common_data() to include 1000BASE-X setup.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>