mtd: nand: Add+use mtd_to/from_nand and nand_get/set_controller_data

These functions are part of the Linux 4.6 sync.  They are being added
before the main sync patch in order to make it easier to address the
issue across all NAND drivers (many/most of which do not closely track
their Linux counterparts) separately from other merge issues.

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

fs/yaffs2/yaffs_uboot_glue.c

@@ -190,7 +190,7 @@ void cmd_yaffs_devconfig(char *_mp, int flash_dev,
 		goto err;
 	}
 
-	chip =  mtd->priv;
+	chip =  mtd_to_nand(mtd);
 
 	/* Check for any conflicts */
 	yaffs_dev_rewind();