TBB: Add an IO abstraction layer to load encrypted firmwares ...

TBBR spec advocates for optional encryption of firmwares (see optional
requirement: R060_TBBR_FUNCTION). So add an IO abstaction layer to
support firmware decryption that can be stacked above any underlying IO/
packaging layer like FIP etc. It aims to provide a framework to load any
encrypted IO payload.

Also, add plat_get_enc_key_info() to be implemented in a platform
specific manner as handling of encryption key may vary from one platform
to another.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: I9892e0ddf00ebecb8981301dbfa41ea23e078b03