clocksource driver may use sched_clock_register
to resigter itself as a sched_clock source.
Export it to support building such driver
as module, like timer-mediatek.c
Bug: 161675989
Signed-off-by: Chun-Hung Wu <chun-hung.wu@mediatek.com>
Change-Id: I610b48abe1d2ec612023173bc847994c34703f56