tee: add a stub for tee_find_device

Add stub for tee_find_device function when CONFIG_TEE is not activated
to simplify the caller code.

This patch allows to remove the CONFIG_IS_ENABLED(OPTEE) tests
for stm32 platform.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Acked-by: Etienne Carriere <etienne.carriere@inaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>