cpuidle: Skip governor when only one idle state is available

In the Linux kernel, the following vulnerability has been resolved:

cpuidle: Skip governor when only one idle state is available

On certain platforms (PowerNV systems without a power-mgt DT node),

cpuidle may register only a single idle state. In cases where that

single state is a polling state (state 0), the ladder governor may

incorrectly treat state 1 as the first usable state and pass an

out-of-bounds index. This can lead to a NULL enter callback being

invoked, ultimately resulting in a system crash.

[ 13.342636] cpuidle-powernv : Only Snooze is available

[ 13.351854] Faulting instruction address: 0x00000000

[ 13.376489] NIP [0000000000000000] 0x0

[ 13.378351] LR [c000000001e01974] cpuidle_enter_state+0x2c4/0x668

Fix this by adding a bail-out in cpuidle_select() that returns state 0

directly when state_count <= 1, bypassing the governor and keeping the

tick running.