ARM: local timers: reserve local_timer_register() to SMP

When running an SMP_ON_UP enabled kernel on UP, or with nosmp
passed to the kernel, we want to be able to detect that a local
timer is not going to be used (local timers are only used on
SMP platforms), so we could register it as a global timer instead.

Return -ENXIO when the above case is detected.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>