dpll: add frequency monitoring callback ops

dpll: add frequency monitoring callback ops

Add new callback operations for a dpll device:
- freq_monitor_get(..) - to obtain current state of frequency monitor
  feature from dpll device,
- freq_monitor_set(..) - to allow feature configuration.

Add new callback operation for a dpll pin:
- measured_freq_get(..) - to obtain the measured frequency in mHz.

Obtain the feature state value using the get callback and provide it to
the user if the device driver implements callbacks. The measured_freq_get
pin callback is only invoked when the frequency monitor is enabled.
The freq_monitor_get device callback is required when measured_freq_get
is provided by the driver.

Execute the set callback upon user requests.

Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20260402184057.1890514-3-ivecera@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>