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: 1447864b ("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>

arch/mips/kernel/smp-cps.c

@@ -424,9 +424,11 @@ static void cps_shutdown_this_cpu(enum cpu_death death)
 			wmb();
 		}
 	} else {
-		pr_debug("Gating power to core %d\n", core);
-		/* Power down the core */
-		cps_pm_enter_state(CPS_PM_POWER_GATED);
+		if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
+			pr_debug("Gating power to core %d\n", core);
+			/* Power down the core */
+			cps_pm_enter_state(CPS_PM_POWER_GATED);
+		}
 	}
 }