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>