This updates the optional (non-default!) NAND support for the
DaVinci DM6446 EVM:

 - include MTD partitioning, defaulting to what Linux uses

 - use a flash-based BBT, which among other things speeds bootup

This matches code that's now queued for mainline Linux, and might
even merge in an upcoming 2.6.30-rc; and the MTIDS are set up so
that the U-Boot $mtdparts environment variable can be passed as-is
on the kernel command line as a cmdlinepart override.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>