arm: u8500_hotplug: various changes yet again

- fix cpu not offlining
- shut up pr_info
- increase up_load to 50
- convert ktime to useconds for better accuracy
- don't add down_timer if sampling time is not reached
- re-calculate online cpus at the start of every hotplug function
- some organisation
- create cpu online/offline functions
- online cpu if load is very high

Change-Id: Id57eedd6ad4c0173eabd506948c5c3fba847543f