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/platform.mk b/plat/nvidia/tegra/platform.mk
index b429eb7..0917d87 100644
--- a/plat/nvidia/tegra/platform.mk
+++ b/plat/nvidia/tegra/platform.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2015-2019, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -68,7 +68,6 @@
 # o resolve undefined symbols to el3_panic
 # o include only required sections
 TF_LDFLAGS	+= --diag_suppress=L6314,L6332 --no_scanlib --callgraph
-TF_LDFLAGS	+= --unresolved=el3_panic
 TF_LDFLAGS	+= --keep="*(__pubsub*)" --keep="*(rt_svc_descs*)" --keep="*(*cpu_ops)"
 ifeq (${ENABLE_PMF},1)
 TF_LDFLAGS	+= --keep="*(*pmf_svc_descs*)"