net/fm: add 2.5G SGMII support

As auto-negotiation is not supported for 2.5G SGMII, we need
to add a new type PHY_INTERFACE_MODE_SGMII_2500 to differentiate
SGMII-1G and SGMII-2.5G with different setting for auto-negotiation.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>