ARM: hisi: fix building without CONFIG_HOTPLUG_CPU

The hisi SMP code always uses the hi3xxx_set_cpu() function
defined in the hotplug.c file, so we cannot build without
this when CONFIG_SMP is enabled. This patch slightly restructures
the code so we always build the parts of hotplug.c that we need
but just leave out the CPU disable logic if CONFIG_HOTPLUG_CPU
is turned off.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>