drivers: optee: use flexible-array member instead of zero-length array

Use flexible-array member introduced in C99 instead of zero-length
array. Most of zero-length array was already taken care in previous
patch [1]. Now modified few more cases which were not handled earlier.

[1]. https://patchwork.kernel.org/patch/11394197/



Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>