B4860QDS: Enable enet port as per fsl_b4860_serdes2 string in hwconfig

B4860QDS: Enable enet port as per fsl_b4860_serdes2 string in hwconfig

In B4860QDS board SerDes2 lanes EFGH either go to SFP or AMC riser card
slot2 so either DTSEC3/DTSEC4 or TGEC1/TGEC2 should be accessible. This
Patch enables DTSEC3/DTSEC4 or TGEC1/TGEC2 on bases of user specified
string fsl_b4860_serdes2:sfp_amc=amc or fsl_b4860_serdes2:sfp_amc=sfp
respectively in hwconfig.

Signed-off-by: Suresh Gupta <suresh.gupta@freescale.com>
Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>