Updated documentation

- Documentation reflects the current state of OP-TEE.
- Added empty sections showing what we intend to add to the
  documentation.
- Added yEd (http://www.yworks.com/) files and the images generated by
  the tool.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>