How many nodes can you install SSRS on when using basic availability groups?
A SSRS scale-out deployment is an Enterprise Edition ONLY feature.  

I don't like providing link-only answers, but this alternative approach outlined by Rodney Landrum, is much too long to copy/paste into an answer, and I suspect his blog entry will be around for quite some time as it's hosted by Red-Gate.

Long story short, you can't approach this via a scale-out deployment, but there is an alternative solution available if you don't mind some additional administration and automation required to make it happen.