ARM: uniphier: only select TWD for SMP

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)

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>