CONFIG_SCHED_CPULOAD_EXTCLK doesn't actually require tickless mode.
As long as the platform provides external call to nxsched_process_cpuload(),
it will work in either tickless or ticking mode.
Removed Kconfig dependency.
Instead, CONFIG_SCHED_CPULOAD_SYSCLK does require ticking mode to work,
as documented in CONFIG_SCHED_CPULOAD help text.
Added the dependency to Kconfig also.