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>