I found it finally.
Follow the steps given below.
Step1 - First find your machine's public IP address.
Step2 - Go to your EC2 instance and from your EC2 instance go to security groups.
Step3 - In your security groups find your database group id, group name and vpc id and select it.
Step4 - On selecting it you will find "Description, Inbound, Outbound, Tags" these four tabs.
Step5 - Select Inbound tab and go to edit and then click on add rule.
Step6 - Keep "Type, Protocol, Port Range, Source" same as above , in IP address section add your machine's "public" IP address and then save.
Step7 - Refresh EC2 instance and your MY SQL RDS database and after 5-10 minutes try reconnecting from your MYSQL Workbench.
It worked for me.