86xx: remove the unused ddr_enable_ecc in the board file

The DDR controller of 86xx processors have the ECC data init
feature, and the new DDR code is using the feature, we don't
need the way with DMA to init memory again.

Signed-off-by: Dave Liu <daveliu@freescale.com>
Acked-by: Kumar Gala <kumar.gala@freescale.com>