I can now get the Registration ID from the mobile and the push notification is working properly. I can access the list with the help of REST APIs but what I want to know is how to access the Registration ID list in the Bluemix itself where I'm not creating any tag.
As you stated above, with the Bluemix Push Notification Service you can use REST APIs in order to gather the deviceIds from registered devices. We also provide a document that allows you to test these APIs directly in the browser:
https://mobile.ng.bluemix.net/imfpushrestapidocs/#!/devices/get_apps_applicationId_devices
Currently we do not provide a list of registered deviceIDs via the Bluemix dashboard, but you can use the Monitoring tab in the Push Notification dashboard to gather some statistics on the number of registered devices.