drivers: counter: add API to query counter frequency
drivers: counter: add API to query counter frequency

Add API to get the base counter frequency.

Signed-off-by: Peter A. Bigot <650538893330be855f76604594b4820eb226769e@pabigot.com>

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