kernel: idle: fix builds with PM but no system clock

PM depends on SYS_CLOCK_EXISTS in Kconfig but several boards have
Kconfig overrides that allow the dependency to be ignored, so
CONFIG_PM=y even though CONFIG_SYS_CLOCK_EXISTS=n.  Fix the code so
that the true dependency is reflected in the generated code.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>