Bring IO storage dummy driver

Allow to handle cases where some images are pre-loaded (by debugger for
instance) without introducing many switches in files calling load_* functions.

Fixes: arm-software/tf-issues#398
Signed-off-by: Gerald Lejeune <gerald.lejeune@st.com>
diff --git a/include/drivers/io/io_storage.h b/include/drivers/io/io_storage.h
index 970ab2c..f7e5b86 100644
--- a/include/drivers/io/io_storage.h
+++ b/include/drivers/io/io_storage.h
@@ -43,6 +43,7 @@
 	IO_TYPE_INVALID,
 	IO_TYPE_SEMIHOSTING,
 	IO_TYPE_MEMMAP,
+	IO_TYPE_DUMMY,
 	IO_TYPE_FIRMWARE_IMAGE_PACKAGE,
 	IO_TYPE_MAX
 } io_type_t;