Blackfin: unify sched_clock() handling between clock sources

Currently sched_clock() is only defined when using CYCLES as a clock
source.  Declare sched_clock() in common code and mark it with notrace to
prevent invoking sched_clock() recursively (because ftrace uses
sched_clock() to record time).

Signed-off-by: Yi Li <yi.li@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>