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>