config_cpu: set ondemand governor only if available

Nowadays we enable and use the ondemand governor by default in
our kernel configuration. More recent systems providing Intel
Haswell CPUs (e.g. the i7-3520M CPU as present in the Lenovo X230
laptops as well as the CPU present in IBM flex chassis) use
pstates/intel_pstate and disable the ondemand governor at all, so
only performance and powersave are available (unless you set
intel_pstate=disable at boot time).

Also see https://plus.google.com/+TheodoreTso/posts/2vEekAsG2QT
Thanks to Yves-Alexis Perez for the pointer.