Build: Merge tfm_partition_defs into tfm_config

The purpose of tfm_partition_defs is to expose the partition related
compile definitions, which can easily be covered by tfm_config.
So merge tfm_partition_defs into tfm_config to reduce the number of
CMake targets.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I8ac9983ece670383183459554c9a22e1e8e22d66
diff --git a/toolchain_IARARM.cmake b/toolchain_IARARM.cmake
index 3b83be2..75ce694 100644
--- a/toolchain_IARARM.cmake
+++ b/toolchain_IARARM.cmake
@@ -146,7 +146,7 @@
     target_link_libraries(${target}_scatter
         platform_region_defs
         psa_interface
-        tfm_partition_defs
+        tfm_config
     )
 
     target_compile_options(${target}_scatter