clocksource/drivers/arm_arch_timer:: Mark cyclecounter __ro_after_init

clocksource/drivers/arm_arch_timer:: Mark cyclecounter __ro_after_init

The object cyclecounter of type cyclecounter is only modified during
initialization in arch_counter_register. So it can be marked
__ro_after_init.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Cc: mark.rutland@arm.com
Cc: keescook@chromium.org
Cc: marc.zyngier@arm.com
Cc: daniel.lezcano@linaro.org
Cc: julia.lawall@lip6.fr
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1486840818-22214-1-git-send-email-bhumirks@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>