This makes uniphier behave like all the other platforms that
support TWD, and only select this driver when SMP is enabled.
Without this, we get a compile error on UP builds:
arch/arm/kernel/smp_twd.c: In function 'twd_local_timer_of_register':
arch/arm/kernel/smp_twd.c:391:20: error: 'setup_max_cpus' undeclared (first use in this function)