Enabling SYS_POWER_MANAGEMENT results in a linker error.

[mnkp]

When SYS_POWER_MANAGEMENT Kconfig option is enabled via menuconfig for hello_world sample application on any of the ARM boards (and likely on others) the linker will generate the following error:

zephyr/kernel/libkernel.a(idle.c.obj): In function `_sys_power_save_idle_exit':
zephyr/kernel/idle.c:115: undefined reference to `z_clock_idle_exit'
collect2: error: ld returned 1 exit status

Zephyr's Power Management documentation is ambiguous on the issue however, enabling the power management system seems to makes sense only if kernel operates in Tickless Idle mode. That's also how the source code was written. The z_clock_idle_exit function is compiled only if TICKLESS_IDLE Kconfig option is enabled.

To ensure the compilation is successful SYS_POWER_MANAGEMENT should select TICKLESS_IDLE Kconfig option.

Also Zephyr's "Power Management" documentation should be updated to clearly state that any of Zephyr's power management features works only in Tickless Idle mode.