mtd: add datasheet's ECC information to nand_chip{}

1.) Why add the ECC information to the nand_chip{} ?
   Each nand chip has its requirement for the ECC correctability, such as
   "4bit ECC for each 512Byte" or "40bit ECC for each 1024Byte".
   This ECC info is very important to the nand controller, such as gpmi.

   Take the Micron MT29F64G08CBABA for example, its geometry is
   8KiB page size, 744 bytes oob size and it requires 40bit ECC per 1KiB.
   If we do not provide the ECC info to the gpmi nand driver, it has to
   calculate the ECC correctability itself. The gpmi driver will gets the 56bit
   ECC for per 1KiB which is beyond its BCH's 40bit ecc capibility.
   The gpmi will quits in this case. But in actually, the gpmi can supports
   this nand chip if it can get the right ECC info.

2.) about the new fields.
   The @ecc_strength_ds stands for the ecc bits needed within the @ecc_step_ds.
   The two fields should be set from the nand chip's datasheets.

   For example:
	"4bit ECC for each 512Byte" could be:
		@ecc_strength_ds = 4, @ecc_step_ds = 512.
	"40bit ECC for each 1024Byte" could be:
		@ecc_strength_ds = 40, @ecc_step_ds = 1024.

3.) Why do not re-use the @strength and @size in the nand_ecc_ctrl{}?
   The @strength and @size in nand_ecc_ctrl{} is used by the nand controller
   driver, while the @ecc_strength_ds and @ecc_step_ds are get from the datasheet.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Reviewed-and-tested-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>