psci: add CPU_IDLE dependency

I ran into a build error for the psci_checker:

drivers/firmware/psci_checker.o: In function `psci_checker':
psci_checker.c:(.init.text+0x528): undefined reference to `cpuidle_devices'

As far as I can tell, this is simply a very rare combination of options,
but the problem has existed since the code was initially added.
Adding a Kconfig dependency makes it build properly.

Fixes: ea8b1c4a ("drivers: psci: PSCI checker module")
Acked-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>