OP-TEE design document

- Adds documentation covering platform initalization, SMC handling, TEE
  SMC Interface, thread handling and MMU.
- Adds empty sections showing what we intend to add to the
  documentation.
- Adds yEd (http://www.yworks.com/) files and the images generated by
  the tool.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
which used used ==> which is used