sys_clock: Explicitly set constants sign
sys_clock: Explicitly set constants sign

Explicitly setting these constants as unsigned.

MISRA rule 10.8

Signed-off-by: Flavio Ceolin <979b9165500b0741b9d0500e2efd74fc1547bff7@intel.com>

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