mtd: move definitions to enlarge their range

Some helpers might be useful in a future 'mtd' U-Boot command to parse
MTD device list.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
diff --git a/include/linux/mtd/mtd.h b/include/linux/mtd/mtd.h
index 272c646..b8c2c3f 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -533,6 +533,12 @@
 int add_mtd_partitions(struct mtd_info *, const struct mtd_partition *, int);
 int del_mtd_partitions(struct mtd_info *);
 
+struct mtd_info *__mtd_next_device(int i);
+#define mtd_for_each_device(mtd)			\
+	for ((mtd) = __mtd_next_device(0);		\
+	     (mtd) != NULL;				\
+	     (mtd) = __mtd_next_device(mtd->index + 1))
+
 int mtd_arg_off(const char *arg, int *idx, loff_t *off, loff_t *size,
 		loff_t *maxsize, int devtype, uint64_t chipsize);
 int mtd_arg_off_size(int argc, char *const argv[], int *idx, loff_t *off,