nand: Embed mtd_info in struct nand_chip

nand_info[] is now an array of pointers, with the actual mtd_info
instance embedded in struct nand_chip.

This is in preparation for syncing the NAND code with Linux 4.6,
which makes the same change to struct nand_chip.  It's in a separate
commit due to the large amount of changes required to accommodate the
change to nand_info[].

Signed-off-by: Scott Wood <oss@buserror.net>