Platform: Fix MUSCA_A boot issue

Improper preprocessors checking caused malfunction of TFM_LINK_ macros,
which leads to a boot failure on MUSCA_A boards. Correct it by adding
TFM_LVL checking since TFM_LVL is not defined while BL2 is compiling.

Change-Id: Iaffdb07aec446c7fad3fff864871c9447028bfe2
Signed-off-by: Edison Ai <edison.ai@arm.com>
diff --git a/platform/include/tfm_plat_defs.h b/platform/include/tfm_plat_defs.h
index 748b2e4..487bba5 100644
--- a/platform/include/tfm_plat_defs.h
+++ b/platform/include/tfm_plat_defs.h
@@ -25,7 +25,7 @@
     TFM_PLAT_ERR_FORCE_INT_SIZE = INT_MAX
 };
 
-#if defined(TFM_PSA_API) && (TFM_LVL != 1)
+#if defined(TFM_PSA_API) && defined(TFM_LVL) && (TFM_LVL != 1)
 
 /*!
  * \def TFM_LINK_SET_RO_IN_PARTITION_SECTION(TFM_PARTITION_NAME)