bcm968380gerg: remove CONFIG_SYS_NAND_DRIVER_ECC_LAYOUT

This board define the flag CONFIG_SYS_NAND_DRIVER_ECC_LAYOUT
but it's a mistake. It's a workaround for an issue in nand core.
This issue was fixed by the commit 5f626e78 ("mtd: nand: raw:
Fix CONFIG_SYS_NAND_DRIVER_ECC_LAYOUT behavior"). Now, this flag
break the nand on this board, so we simply remove it.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>