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>
diff --git a/board/st/common/stm32mp_mtdparts.c b/board/st/common/stm32mp_mtdparts.c
index 8b636d6..1887842 100644
--- a/board/st/common/stm32mp_mtdparts.c
+++ b/board/st/common/stm32mp_mtdparts.c
@@ -119,8 +119,7 @@
 	}
 
 #ifdef CONFIG_STM32MP15x_STM32IMAGE
-	if (!serial && CONFIG_IS_ENABLED(OPTEE) &&
-	    tee_find_device(NULL, NULL, NULL, NULL))
+	if (!serial && tee_find_device(NULL, NULL, NULL, NULL))
 		tee = true;
 #endif