platform: stm: only use plat_data_ptr with isolation level 2

Fix the -Wunused-variable warning:
platform/ext/target/stm/common/stm32h5xx/secure/tfm_hal_isolation.c:424:29:
warning: unused variable 'plat_data_ptr' [-Wunused-variable]
  424 |     struct platform_data_t *plat_data_ptr;
      |                             ^~~~~~~~~~~~~

Change-Id: I8a48cf15302d64ba78d0aec891ec36b42a952cbd
Signed-off-by: Stefan Gloor <[email protected]>
(cherry picked from commit 50da8e161cc1d2171565e94ae5846eea95a5d6bd)
Signed-off-by: Tim Pambor <[email protected]>

Author: stgloorious

platform/ext/target/stm/common/stm32h5xx/secure/tfm_hal_isolation.c

@@ -421,8 +421,8 @@ FIH_RET_TYPE(enum tfm_hal_status_t) tfm_hal_bind_boundary(
     bool ns_agent;
     uint32_t partition_attrs = 0;
     const struct asset_desc_t *p_asset;
-    struct platform_data_t *plat_data_ptr;
 #if TFM_ISOLATION_LEVEL == 2
+    struct platform_data_t *plat_data_ptr;
     struct mpu_armv8m_region_cfg_t localcfg;
 #endif