mtd: gpmi: rewrite the gpmi_ecc_write_oob() to support the jffs2

When we use the ECC info which is get from the nand chip's datasheet,
we may have some freed oob area now.

This patch rewrites the gpmi_ecc_write_oob() to implement the ecc.write_oob().
We also update the comment for gpmi_hw_ecclayout.

Yes! We can support the JFFS2 for the SLC nand now.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>