April 2nd, 2018 Freedom E SDK Release

We haven't had a release for a while, so I don't know the change list.

Update SDK For E31/E51 Coreplex IP Evaluation

Merge pull request sifive#53 from sifive/counters_demo

Add Performance Counter Demo

Merge pull request sifive#50 from sifive/common_plic

FE-203 Move plic_driver code to a common location.