Fix Linux task preemption on Versatile board

Backport from master:

  Recent versions of the Linux kernel will not preempt CPU-intensive
  tasks unless the clock used by sched_clock() works.  On -M versatilepb
  that's the 24MHz timer in the system controller.  It's a very simple
  timer, so implement it.

Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>