kernel: minor improve in SYS_CLOCK_HW_CYCLES_PER_SEC help description
kernel: minor improve in SYS_CLOCK_HW_CYCLES_PER_SEC help description

Minor improvement in the help text description of Kconfig option
SYS_CLOCK_HW_CYCLES_PER_SEC, clarifying that the option can be
defined in either SOC or Board Kconfig file.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

galak/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr,explora26/zephyr,kraj/zephyr,finikorg/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,ldts/zephyr,punitvara/zephyr,punitvara/zephyr,kraj/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,nashif/zephyr,kraj/zephyr,Vudentz/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,punitvara/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,galak/zephyr,nashif/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,Vudentz/zephyr