fix(tc0): set cactus-tertiary vcpu count to 1

Third instance of cactus is a UP SP. Set its vcpu count to 1.
Signed-off-by: Arunachalam Ganapathy <arunachalam.ganapathy@arm.com>
Change-Id: I34b7feb2915e6d335e690e89dea466e75944ed1b

plat/arm/board/tc0/fdts/tc0_spmc_manifest.dts

 /*
- * Copyright (c) 2020, Arm Limited. All rights reserved.
+ * Copyright (c) 2020-2021, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -40,7 +40,7 @@
 			is_ffa_partition;
 			debug_name = "cactus-tertiary";
 			load_address = <0xfe200000>;
-			vcpu_count = <8>;
+			vcpu_count = <1>;
 			mem_size = <1048576>;
 		};
 	};