clocksource: arm_arch_timer: Hide eventstream Kconfig on non-ARM

Pavel Machek reports that this config is exposed on x86 where the
ARM architected timers aren't even present. Make it depend on the
ARM architected timers being selected so that non-ARM builds
aren't asked about it.

Reported-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>