Define and use a DaVinci-specific NAND platform_data structure,
instead of reusing generic NOR platform_data.  Improvements:

 - ALE and CLE bits are no longer hard-wired
 - ECC mode is in board-specific data, not a Kconfig option
 - Supports board-specific options like BUSWIDTH_16 and FLASH_BBT

Plus:

 - Don't force the use of MTD partitioning
 - Make that platform data be optional
 - Now pdev->id specifies chipselect (must be zero for now)
 - Add missing iounmap calls on probe failure

And corresponding updates for NAND support on the two EVM boards
that now have it.  Note that dm355 evm boards will fail cleanly
on boot now, pending 4-bit ECC support.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>