umm, the comment at the top of linux-4.19.53/drivers/cpuidle/cpuidle-arm.c says this file is the driver, and it is present. However,
root@OpenWrt:~# cat /sys/devices/system/cpu/cpuidle/current_driver none
and from my build log:
* * ARM CPU Idle Drivers * Generic ARM/ARM64 CPU idle Driver (ARM_CPUIDLE) [N/y/?] (NEW)
which looks like it didn't get selected.
EDIT: but that's no different than my last 4.14 compile and I don't know what current_driver gives for 4.14 so I'm back to clueless