optee: rename rpc_arg_count to rpc_param_count

Renames the field rpc_arg_count in struct optee to rpc_param_count.
Function parameter names and local variables are also renamed to match.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>