Add support for Cortex-M85

Rely on CMSIS code for initializing Cortex-M85 devices.

Fallback to mcpu=cortex-m55 for gcc until support for cortex-m85 is
available.

Change-Id: I8b47563c3f0f44e35735a569f2abf8a308948e67
diff --git a/targets/corstone-310/platform.scatter b/targets/corstone-310/platform.scatter
index f23f3a3..94d34e1 100644
--- a/targets/corstone-310/platform.scatter
+++ b/targets/corstone-310/platform.scatter
@@ -115,8 +115,8 @@
     {
         *.o (RESET, +First)
         *(InRoot$$Sections)
-        startup_ARMCM55.o
-        system_ARMCM55.o
+        startup_ARMCM85.o
+        system_ARMCM85.o
         target.o
         .ANY1 (+RO +RW +ZI)
     }