mtd: spi: Add 'struct spi_flash {' to the code

At present spi_flash is defined to be spi_nor which is confusing since it
is not possible to find the 'spi_flash' by normal text search. Add a
comment to help with this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/linux/mtd/spi-nor.h b/include/linux/mtd/spi-nor.h
index 709b49d..f9964a7 100644
--- a/include/linux/mtd/spi-nor.h
+++ b/include/linux/mtd/spi-nor.h
@@ -246,7 +246,13 @@
  */
 struct flash_info;
 
-/* TODO: Remove, once all users of spi_flash interface are moved to MTD */
+/*
+ * TODO: Remove, once all users of spi_flash interface are moved to MTD
+ *
+ * struct spi_flash {
+ *	Defined below (keep this text to enable searching for spi_flash decl)
+ * }
+ */
 #define spi_flash spi_nor
 
 /**