mtd: use bbm.h in nand.h

This consolidates common code in nand.h and bbm.h. The
comments and data structures were the same, this keeps
the comment from nand.h as it fits 80 columns, while the one
in bbm.h did not.

Signed-off-by: Alessandro Rubini <rubini@unipv.it>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>