sched_clock: Add temporary asm/sched_clock.h

Some new users of the ARM sched_clock framework are going through
the arm-soc tree. Before 38ff87f7 (sched_clock: Make ARM's
sched_clock generic for all architectures, 2013-06-01) the header
file was in asm, but now it's in linux. One solution would be to
do an evil merge of the arm-soc tree and fix up the asm users,
but it's easier to add a temporary asm header that we can remove
along with the few stragglers after the merge window is over.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>