clocksource: arch_timer: Export arch_counter_get_cntvct()

get_cycles() on arm64 is defined to be arch_counter_get_cntvct().
Export this function to modules so that modules can use
get_cycles() appropriately.

Change-Id: I3e66af01cbd4f2631637ccfba94e1fe589625b4e
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>