optee: add missing description of RPC argument reference

Adds missing inline description comment for RPC optional arguments
reference.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/lkml/202310192021.fvb6JDOY-lkp@intel.com/


Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>