mtd: nand: Sync with Linux v4.6

Updates the NAND code to match Linux v4.6.  The previous sync was from
Linux v4.1 in commit d3963721.

Note that none of the individual NAND drivers tracked Linux closely
enough to be synced themselves, other than manually applying a few
cross-tree changes.

Signed-off-by: Scott Wood <oss@buserror.net>
Tested-by: Heiko Schocher <hs@denx.de>