optee_design.md: add picture to MMU section

xlat_table.png is generated with the command:
    dot -Tpng xlat_table.dot > xlat_table.png

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Fixes in trace refactoring

- Deprecates ATAMSG
- trace_ext.c is arm32 specific
- tee_kta_trace.h and trace_ta.h are generic
- Default level is now 1 (was 2)

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>