When I attempted to install Windows 7 Pro on a family member's new build, I came across this same issue. So common is this complaint that Asrock has a site dedicated to the problem. I'm reasonably sure that the other vendors do as well: http://www.asrock.com/microsite/win7install/
Simplest work-around: If your motherboard has a PS/2 port (many do) then you could simply swap between a PS/2 mouse & keyboard to get Windows 7 installed. I'm not sure if a USB-to-PS/2 adapter would work but probably. Another solution which I never thought to try is to plug your storage into a different non-skylake build, install Windows 7, and then transport that storage back to your new Skylake build. I can't see why that wouldn't work, but I've...