drivers: tee: optee: handlde GET_THREAD_COUNT call

Optee driver should handle GET_THREAD_COUNT call. This limitation is
set in Optee-OS during build and limits number of symultanious calls
to optee. Add sym to handle this limitation.

Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com>