davinci_nand: cleanup I (minor)

Minor cleanup for DaVinci NAND code:

 - Use I/O addresses from nand_chip; CONFIG_SYS_NAND_BASE won't
   be defined when there are multiple chipselect lines in use
   (as with common 2 GByte chips).

 - Cleanup handling of EMIF control registers
    * Only need one pointer pointing to them
    * Remove incorrect and unused struct supersetting them

 - Use the standard waitfunc; we don't need a custom version

 - Partial legacy cleanup:
    * Don't initialize every board like it's a DM6446 EVM
    * #ifdef a bit more code for BROKEN_ECC

Sanity checked with small page NAND on dm355 and dm6446 EVMs;
and large page on dm355 EVM (packaged as two devices, not one).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Scott Wood <scottwood@freescale.com>