Adding fixed sdram setting for cornet_ds board

800, 900, 1000, 1200MT/s data rate parameters are added for fixed sdram
setting. SPD based parameters and fixed parameters can be toggled by hwconfig.
To use fixed parameters,

hwconfig=fsl_ddr:sdram=fixed

To use SPD parameters,

hwconfig=fsl_ddr:ctlr_intlv=cacheline,bank_intlv=cs0_cs1

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>