sched: sched_clock() clocksource handling.

There are presently a number of issues and limitations with how the
clocksource and sched_clock() interaction works today. Configurations
tend to be grouped in to one of the following:

    - Platform provides a clocksource unsuitable for sched_clock()
      and prefers to use the generic jiffies-backed implementation.

    - Platform provides its own clocksource and sched_clock() that
      wraps in to it.

    - Platform uses a generic clocksource (ie, drivers/clocksource/)
      combined with the generic jiffies-backed sched_clock().

    - Platform supports multiple sched_clock()-capable clocksources.