clocksource: arch_timer: Pass clock event to set_mode callback

There isn't any reason why we don't pass the event here and we'll
need it in the near future for memory mapped arch timers anyway.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <Marc.Zyngier@arm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>