Dell XPS 9560: Add info to README and adjust configuration

[Lyndeno]

Description of changes

Added information about hardware functionality and solutions I have discovered:

• Firmware updates with fwupd
• System control with libsmbios
• TPM 2.0 functions not working (with solutions)
• Note on secure boot functionality
• Note on using the TPM to unlock root drive

And some configuration changes:

• Do not use lib.mkDefault for the GPU IDs. User should not need to change this and this will prevent accidentally changing it.
• Import the common/cpu/intel/kaby-lake configuration for intel and bumblebee configurations

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

[Lyndeno]

Is there anything else needed from my end for this PR to be merged?