davinci: DM644x: NAND: update partitioning

davinci: DM644x: NAND: update partitioning

Update NAND partitioning for the dm6446 evm, unmasking the hidden
data at the beginning and letting the kernel be updated from Linux.

 - This is boot-compatible with TI's software (U-Boot 1.20 and both
   the 2.6.10 and 2.6.18 kernels), in terms of startup and loading
   kernels from flash.

 - In the same way, it's also boot-compatible with mainline U-Boot,
   which stores U-Boot params in block 0 not block 16.

 - It's not quite compatible with systems that previously used NAND
   partitions to hold (filesystem) data.  The compatibilities are a
   bit different based on which kernel was used previously
     + Users of TI/MV kernels no longer see mtd2 "params"
       (mainline u-boot env is in a different place)
	* Filesystem is now mtd2 ... vs mtd3
     + Users of GIT kernels now see mtd0 and mtd1 partitions
	* Filesystem partition starts 640 KBytes earlier
	* Filesystem is now mtd2 ... vs mtd0
     * Linux now *uses* the flash-resident BBT
	* Removes annoying slowdown/hiccup during boot
	* Potentially ~64KB less space available with TI/MV kernels

If you *used* NAND partitions from Linux, there is no solution that's
fully compatible with all previous kernels in those respects ... ergo
this "best compromise".  It'd be good to back back up the filesystem
data; or, carry your own backwards-compatibility patch for awhile.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>