Cannot create only IAM policy with cloudformation

You probably want to create an AWS::IAM::ManagedPolicy if you just want a standalone policy.

http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-iam-managedpolicy.html

From the documentation:

AWS::IAM::ManagedPolicy creates an AWS Identity and Access Management (IAM) managed policy for your AWS account, which you can use to apply permissions to IAM users, groups, and roles.

Here's an example:

Resources:
  CreateTestDBPolicy: 
    Type: AWS::IAM::ManagedPolicy
    Properties: 
      Description: "Policy for creating a test database"
      Path: "/"
      PolicyDocument: 
      Version: "2012-10-17"
      Statement: 
        - 
          Effect: "Allow"
          Action: "rds:CreateDBInstance"
          Resource: "*"

This will resolve your issue.