mtd: Add some fallbacks for add/del_mtd_device

This allows using these functions without ifdefs. OneNAND depends on MTD,
so this ifdef was redundant in the first place.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
diff --git a/include/linux/mtd/mtd.h b/include/linux/mtd/mtd.h
index 09f5269..7a66c7a 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -552,8 +552,20 @@
 
 #ifdef __UBOOT__
 /* drivers/mtd/mtdcore.h */
+#if CONFIG_IS_ENABLED(MTD)
 int add_mtd_device(struct mtd_info *mtd);
 int del_mtd_device(struct mtd_info *mtd);
+#else
+static inline int add_mtd_device(struct mtd_info *mtd)
+{
+	return -ENOSYS;
+}
+
+static inline int del_mtd_device(struct mtd_info *mtd)
+{
+	return -ENOSYS;
+}
+#endif
 
 #ifdef CONFIG_MTD_PARTITIONS
 int add_mtd_partitions(struct mtd_info *, const struct mtd_partition *, int);