This is though task, witch may require a lot of work to do.
First i thought there have to be way that Android OS hold that kind of data somewhere in storage. So i found this http://android-er.blogspot.com/2009/09/read-android-cpu-info.html. In short you read all the data from manufacturer, but on different phones there might or may not be data you are looking for.
Second thought was you have to count it on your own https://stackoverflow.com/a/27821658/2531209 This might come in handy, but its not perfect since you need to make calculations and you wont receive result in Hz.
Third thought if this is your core of application and you want to support it you may want to create list of all phones and processors architectures (or find one in convenient form) and display associated with it score in GHz.