sched_clock: Make ARM's sched_clock generic for all architectures

sched_clock: Make ARM's sched_clock generic for all architectures

Nothing about the sched_clock implementation in the ARM port is
specific to the architecture. Generalize the code so that other
architectures can use it by selecting GENERIC_SCHED_CLOCK.

Bug 1399318

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
[jstultz: Merge minor collisions with other patches in my tree]
Signed-off-by: John Stultz <john.stultz@linaro.org>
(cherry picked from commit 38ff87f77af0b5a93fc8581cff1d6e5692ab8970)
Signed-off-by: Ajay Nandakumar <anandakumarm@nvidia.com>
Change-Id: Iaac325635344ef4adee8fa0f9ca11aee089bce0f