Add Dell Inspiron 7559 and corresponding Skylake architecture

[ccritter]

Description of changes

• Adds Skylake CPU/GPU config
  • Verified support with vainfo and on Firefox + Chrome hardware acceleration
• Adds Inspiron 7559 entries
  • Uses Nvidia Prime with offloading for support between the onboard and discrete GPU
  • Notes on some optional configuration in the README
  • Tested on fresh NixOS install using Flakes and Plasma DE

Things done

• Tested the changes in your own NixOS Configuration
• Tested the changes end-to-end by using your fork of nixos-hardware and
  importing it via <nixos-hardware> or Flake input

[ccritter]

Shoot, I only just now saw that new common modules are no longer accepted in favor of #992 for upstreaming to NixOS. To be honest, don't really know what that would involve, or what that would mean for this 7559 entry and the common modules it's pointing to. Any documentation for that process?