Tegra: dummy support for the io_storage backend

This patch provides dummy macros and platform files to compile
the io_storage driver backend. This patch is necessary to
remove the "--unresolved=el3_panic" linker flag from Tegra's
makefiles and allow us to revert this workaround, previously
suggested by the ARM toolchain team.

The "--unresolved=el3_panic" flag actually was a big hammer that
allowed Tegra platforms to work with armlink previously but it
masks legit errors with the code as well.

Change-Id: I0421d35657823215229f84231896b84167f90548
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
diff --git a/plat/nvidia/tegra/common/tegra_io_storage.c b/plat/nvidia/tegra/common/tegra_io_storage.c
new file mode 100644
index 0000000..21641aa
--- /dev/null
+++ b/plat/nvidia/tegra/common/tegra_io_storage.c
@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2019, NVIDIA Corporation. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <errno.h>
+#include <plat/common/platform.h>
+
+/*
+ * Return an IO device handle and specification which can be used to access
+ * an image. Use this to enforce platform load policy.
+ *
+ * This function is not supported at this time
+ */
+int plat_get_image_source(unsigned int image_id, uintptr_t *dev_handle,
+			  uintptr_t *image_spec)
+{
+	return -ENOTSUP;
+}