driver: clock_control: Add to nrf clock control calib in progres API

It may be required to get information if NRF LF clock control calibration
is in progress. Some time sensitive operations could benefit from this
information.

The commit adds simple function that provides the information.
The function is nRF platform specific.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>