xtensa: fix !CONFIG_XTENSA_CALIBRATE_CCOUNT build failure

Commits 925f5532 (xtensa: ccount based clockevent implementation) and e3f43291
(xtensa: ccount based sched_clock) introduced users of ccount_freq. This
variable doesn't exist when CONFIG_XTENSA_CALIBRATE_CCOUNT is disabled. Add
ccount_freq definition in this case.

Reported-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>