drivers: litex_timer: Do not hard code tick rate
drivers: litex_timer: Do not hard code tick rate

The litex_timer driver used hard coded tick rate (set to 100 ticks
per second). This commit replaces the fixed value with a call to
system function which takes under account system configuration.

Signed-off-by: Piotr Zięcik <9ef3595b5f0f4fa8d0ceeb52cc0cf0411a0684dd@nordicsemi.no>

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