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>