MIPS: SMP-CPS: fix build error when HOTPLUG_CPU not set

MIPS: SMP-CPS: fix build error when HOTPLUG_CPU not set

When MIPS_CPS=y, MIPS_CPS_PM is not set, HOTPLUG_CPU is not set, and
KEXEC=y, cps_shutdown_this_cpu() attempts to call cps_pm_enter_state(),
which is not built when MIPS_CPS_PM is not set.
Conditionally execute the else branch based on CONFIG_HOTPLUG_CPU
to remove the build error.
This build failure is from a randconfig file.

mips-linux-ld: arch/mips/kernel/smp-cps.o: in function `$L162':
smp-cps.c:(.text.cps_kexec_nonboot_cpu+0x31c): undefined reference to `cps_pm_enter_state'

Fixes: 1447864bee4c ("MIPS: kexec: CPS systems to halt nonboot CPUs")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Dengcheng Zhu <dzhu@wavecomp.com>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Cc: Sergei Shtylyov <sergei.shtylyov@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>