Clocklib: generic clocks management framework

Hi,

Here is second attempt to introduce clocks management framework based on
kobjects. This attempt goes a bit further. I've removed the global
spinlock. All add/remove operations are guarded via built into clocks kset
spinlock. And all per-clock operations are guided by built-in atomic
counter variable. I can split it into counter and per-clock spinlock,
but I think it will be a waste of space.

Also as suggested by Andrew Morton, I've split the set_mode operation
back to enable/disable.

-- 
With best wishes
Dmitry