xilinx: zynqmp: Free allocated field for target variable

When env_set() is called there is no need to allocate memory for variable
which is already saved that's why free it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

board/xilinx/zynqmp/zynqmp.c

@@ -735,6 +735,7 @@ int board_late_init(void)
 			env_targets ? env_targets : "");
 
 	env_set("boot_targets", new_targets);
+	free(new_targets);
 
 	reset_reason();