soc: Add BL31 for Cavium CN81xx Soc

* Add BL31.elf compiled using coreboot toolchain
* Add README, LICENSE and VERSION
* ATF Version 1.5
* Based on commit d48f193d11b4d4dca2675646ad76147f2d4765f2
* https://github.com/9elements/arm-trusted-firmware/commits/coreboot_bl31_blob
* Added here as blob as code is not upstream
* Can be removed as soon as it's upstream

Change-Id: I99128cd05735c6dc429d51d642d6afe3f513be3e
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>

soc/cavium/cn81xx/LICENSE

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+                          License Agreement
+               For Open Source Computer Vision Library
+                       (3-clause BSD License)
+
+Copyright 2018       Facebook, Inc.
+Copyright 2003-2017  Cavium Inc.  <support@cavium.com>
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+  * Redistributions of source code must retain the above copyright notice,
+    this list of conditions and the following disclaimer.
+
+  * Redistributions in binary form must reproduce the above copyright notice,
+    this list of conditions and the following disclaimer in the documentation
+    and/or other materials provided with the distribution.
+
+  * Neither the names of the copyright holders nor the names of the contributors
+    may be used to endorse or promote products derived from this software
+    without specific prior written permission.
+
+This software is provided by the copyright holders and contributors "as is" and
+any express or implied warranties, including, but not limited to, the implied
+warranties of merchantability and fitness for a particular purpose are disclaimed.
+In no event shall copyright holders or contributors be liable for any direct,
+indirect, incidental, special, exemplary, or consequential damages
+(including, but not limited to, procurement of substitute goods or services;
+loss of use, data, or profits; or business interruption) however caused
+and on any theory of liability, whether in contract, strict liability,
+or tort (including negligence or otherwise) arising in any way out of
+the use of this software, even if advised of the possibility of such damage.

soc/cavium/cn81xx/README

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+Fork of https://github.com/ARM-software/arm-trusted-firmware
+Version 1.5 based on commit d48f193d11b4d4dca2675646ad76147f2d4765f2
+Source is available at:
+https://github.com/9elements/arm-trusted-firmware/commits/coreboot_bl31_blob

soc/cavium/cn81xx/VERSION

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+version          : ATF 1.5-117f85dd
+release date     : 07-19-2018
+supported silicon: Cavium CN81XX
+ERRATA           : -
+toolchain version: GCC 6.3.0
+ABI version      : 1.5
+arch             : aarch64
+instruction      : Include into coreboot's CBFS and build with
+                   CONFIG_ARM64_USE_ARM_TRUSTED_FIRMWARE