kernel: idle: fix builds with PM but no system clock
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 <a4112a160b558d5427af084318038e19a95979ee@nordicsemi.no>

nashif/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr