ANDROID: GKI: 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-imx-tpm.c

Bug: 194108974
Signed-off-by: Jindong Yue <jindong.yue@nxp.com>
Change-Id: Id23f3da624a1e70fc1a44daf6f827c03dc1d053d