powerpc/cpuidle: Make it a bool, not a tristate

As pointed out, asm/system.h has empty inline implementations for
update_smt_snooze_delay and pseries_notify_cpuidle_add_cpu, which are
used when CONFIG_PSERIES_IDLE is undefined. Since those two functions
are used in core power architecture functions (store_smt_snooze_delay
at kernel/sysfs.c and smp_xics_setup_cpu at platforms/pseries/smp.c),
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/Kconfig

@@ -122,7 +122,7 @@ config DTL
 	  Say N if you are unsure.
 
 config PSERIES_IDLE
-	tristate "Cpuidle driver for pSeries platforms"
+	bool "Cpuidle driver for pSeries platforms"
 	depends on CPU_IDLE
 	depends on PPC_PSERIES
 	default y