mpc8xxx: Enable ECC on/off control in hwconfig

Add fsl_ddr:ecc=on in hwconfig. If ECC is enabled in board configuration file,
ECC can be turned on/off by this switch. If this switch is omitted, it is ON by
default.

Updated hwconfig calls to use local buffer.

Syntax is
hwconfig=fsl_ddr:ecc=on

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