FROMLIST: time/sched_clock: Export sched_clock_register()

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

Signed-off-by: Chun-Hung Wu <chun-hung.wu@mediatek.com>
Signed-off-by: Walter Chang <walter.chang@mediatek.com>
Acked-by: John Stultz <jstultz@google.com>

Bug: 161675989
Link: https://lore.kernel.org/lkml/20230517022557.24388-2-walter.chang@mediatek.com/


Change-Id: Ib052d1fd7ccf6a7422eb6f1755515e1236285e01
Signed-off-by: Carlos Llamas <cmllamas@google.com>