Add mtk8173.md

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>

devices/mtk8173.md

+# OP-TEE on MediaTek 8173 EVB
+
+# Contents
+1. [Introduction](#1-introduction)
+2. [Regular build](#2-regular-build)
+3. [External guide](#3-external-guide)
+
+# 1. Introduction
+The instructions here will tell how to run OP-TEE using the MTK1873 device.
+
+# 2. Regular build
+Start by following the "Get and build the solution" in the [README.md] file.
+When everything has been built and it is booting up, it will stop in fastboot
+waiting for the binaries. At that point, press reset button and the flashing
+procedure should begin.
+
+# 3. External guide
+A MediaTek engineer (Howard Chen) has put detailed descriptions on how to
+connect the device etc in his [MTK8173 Getting Started] guide. That page also
+contains information about how to upgrade the firmware etc. There is also a
+link for how to run OP-TEE. We **don't** recommend that you follow that, since
+that guide is very old and has been superseded by this particular guide.
+
+[README.md]: https://github.com/OP-TEE/build/README.md
+[MTK8173 Getting Started]: https://github.com/ibanezchen/linux-8173/wiki