SPM: declare third cactus instance as UP SP

The FF-A v1.0 spec allows two configurations for the number of EC/vCPU
instantiated in a Secure Partition:
-A MultiProcessor (MP) SP instantiates as many ECs as the number of PEs.
An EC is pinned to a corresponding physical CPU.
-An UniProcessor (UP) SP instantiates a single EC. The EC is migrated to
the physical CPU from which the FF-A call is originating.
This change permits exercising the latter case within the TF-A-tests
framework.
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I7fae0e7b873f349b34e57de5cea496210123aea0

plat/arm/board/fvp/fdts/fvp_spmc_manifest.dts

@@ -47,7 +47,7 @@
 			is_ffa_partition;
 			debug_name = "cactus-tertiary";
 			load_address = <0x7200000>;
-			vcpu_count = <8>;
+			vcpu_count = <1>;
 			mem_size = <1048576>;
 		};
 	};