mtd: nand: davinci_nand, 4-bit ECC for smallpage

Minimal support for the 4-bit ECC engine found on DM355, DM365,
DA830/OMAP-L137, and similar recent DaVinci-family chips.

This is limited to small-page flash for now; there are some page
layout issues for large page chips.  Note that most boards using
this engine (like the DM355 EVM) include 2GiB large page chips.

Sanity tested on DM355 EVM after swapping the socketed NAND for
a small-page one.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

arch/arm/mach-davinci/include/mach/nand.h

@@ -68,10 +68,14 @@ struct davinci_nand_pdata { /* platform_data */
 
  /* none == NAND_ECC_NONE (strongly *not* advised!!)
  * soft == NAND_ECC_SOFT
- * 1-bit == NAND_ECC_HW
- * 4-bit == NAND_ECC_HW_SYNDROME (not on all chips)
+ * else == NAND_ECC_HW, according to ecc_bits
+ *
+ * All DaVinci-family chips support 1-bit hardware ECC.
+ * Newer ones also support 4-bit ECC, but are awkward
+ * using it with large page chips.
  */
  nand_ecc_modes_t ecc_mode;
+ u8 ecc_bits;
 
  /* e.g. NAND_BUSWIDTH_16 or NAND_USE_FLASH_BBT */
  unsigned options;