drivers: clock_control: Smartbond: Add runtime frequency support

RC32K and RCX low power clocks require runtime calibration to work
correctly.
Frequency of those clock can differ from chip to chip, one constant
value from Kconfig may not be best when low power clock (sourced
from RCX or RC32K) is used for system tick.

This code modifies global z_clock_hw_cycles_per_sec variable that
is used when TIMER_READS_ITS_FREQUENCY_AT_RUNTIME is enabled
in Kconfig.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>