ARM: Make the sched_clock framework mandatory

All sched_clock() providers have been converted to the sched_clock
framework, which also provides a jiffy based implementation for
the platforms that do not provide a counter.

It is now possible to make the sched_clock framework mandatory,
effectively preventing new platforms to add new sched_clock()
functions, which would be detrimental to the single zImage work.

Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>