clocksource/drivers/arm_arch_timer: Fix bogus cpu_all_mask usage

Using cpu_all_mask as target mask for clockevents is wrong as it never can
actually target not possible CPUs. Use cpu_possible_mask instead

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>