documentation/arm_trusted_firmware.md: Fixed maximum debug level
documentation/arm_trusted_firmware.md: Fixed maximum debug level

Both README.md and trace_levels.h say the level ranges from 1 to 4.

Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>
Reviewed-by: Pascal Brand <0b55292d2562685bad4ce52d64f802e8f1cd299e@linaro.org>
Signed-off-by: Olivier Martin <77cccf0d7a72ee0036f6f1a239d5c47ee8799014@labapart.com>

cedric-chaumont-st-dev/optee_os,cedric-chaumont-st-dev/optee_os,Microsoft/optee_os,pascal-brand-st-dev/optee_os,Microsoft/optee_os,cedric-chaumont-st-dev/optee_os,matt2048/optee_os,matt2048/optee_os,matt2048/optee_os,cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os,Microsoft/optee_os,matt2048/optee_os,pascal-brand-st-dev/optee_os,Microsoft/optee_os,pascal-brand-st-dev/optee_os,Microsoft/optee_os,pascal-brand-st-dev/optee_os