cpufreq: add enhanced idle support for atom n2600/n2800 series

Per bugzilla and techpatterns forums, Intel Atom N2600/N2800 series support
C-states that the intel_idle idle module utilizes.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=60201
Techpatterns: http://techpatterns.com/forums/about2306.html