Read CPU IDs from FDT in tests, so they work under FVP.

Bug: 117551008
Change-Id: I14e24e86631f9e7c16ad69decf676b343ac9f252
diff --git a/build/toolchain/embedded.gni b/build/toolchain/embedded.gni
index e74fb6f..462bbfa 100644
--- a/build/toolchain/embedded.gni
+++ b/build/toolchain/embedded.gni
@@ -362,13 +362,13 @@
                              "gic_version",
                              "gicd_base_address",
                              "gicr_base_address",
+                             "max_cpus",
                              "toolchain_args",
                            ])
     cpu = "${invoker.cpu}+fp"
 
     # Nonsense values because they are required but shouldn't be used.
     heap_pages = 0
-    max_cpus = 1
     max_vms = 1
   }
 }