drivers: clock_control: smartbond: Add calibration

RCX and RC32K oscillators are not precisely trimmed.
This code allows to measure actual frequency of those two
oscillators.

Device tree binding were extended to specify calibration
interval. This interval (in seconds) is used to periodically
call work that will perform oscillator frequency measurement.

For XTAL32K settle time can be provided in device tree.
After this time (depending on actual oscillator used)
XTAL32K is assumed to be stable and low power clock driven by
XTAL32K is considered OK for precise usage in bluetooth.

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