time: Define dummy functions for the generic sched clock

When we try to compile a clocksource driver with the COMPILE_TEST option,
we can't select the GENERIC_SCHED_CLOCK because the sched_clock() symbol
will be duplicated with the one defined for the x86.

In order to fix that, we don't select the GENERIC_SCHED_CLOCK in the
driver Kconfig's file but we define some empty functions for the different
symbols in order to prevent the unresolved ones.

This patch fixes the COMPILE_TEST option for the compile test coverage for
the clocksource drivers. Without this patch, we can't add the COMPILE_TEST
option for the clocksource drivers using the GENERIC_SCHED_CLOCK.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>