powerpc/c29xpcie: add DDR ECC on off config setting

c29xpcie REV_A board DDR ECC chip has bad impedance in hardware,
force that kind of board to be DDR ECC off when booting.
Other version board config ECC on/off by hwconfig=fsl_ddr:ecc=on
in uboot enviroment.

Signed-off-by: Po Liu <Po.Liu@freescale.com>
Acked-by: York Sun <yorksun@freescale.com>