SPM: Add owner field to cactus secure partitions

For supporting dualroot CoT for Secure Partitions a new optional field
"owner" is introduced which will be used to sign the SP with
corresponding signing domain. To demonstrate its usage, this patch adds
owners to cactus Secure Partitions.
Signed-off-by: Ruari Phipps <ruari.phipps@arm.com>
Change-Id: I7b760580355fc92edf5402cecc38c38125dc1cae

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

@@ -84,11 +84,13 @@
 		cactus-primary {
 			uuid = <0x1e67b5b4 0xe14f904a 0x13fb1fb8 0xcbdae1da>;
 			load-address = <0x7000000>;
+			owner = "SiP";
 		};
 
 		cactus-secondary {
 			uuid = <0x092358d1 0xb94723f0 0x64447c82 0xc88f57f5>;
 			load-address = <0x7100000>;
+			owner = "Plat";
 		};
 #endif
 	};