mtd: nand: minor davinci_nand cleanup

Make the DaVinci NAND driver require platform_data with
board-specific configuration.  We can't actually do any
kind of sane job of configuring it otherwise.

Also fix the comment about picking the "best" ECC mode.

We can't do those any more; that relied on knowing what kind
of CPU we're using (they don't all support 4-bit ECC), and
current policy is that drivers not have cpu_is_*() checks.

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>