dpll: add phase_offset_avg_factor_get/set callback ops

Add new callback operations for a dpll device:
- phase_offset_avg_factor_get(...) - to obtain current phase offset
  averaging factor from dpll device,
- phase_offset_avg_factor_set(...) - to set phase offset averaging factor

Obtain the factor value using the get callback and provide it to the user
if the device driver implement this callback. Execute the set callback upon
user requests, if the driver implement it.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
v2:
* do not require 'set' callback to retrieve current value
* always call 'set' callback regardless of current value
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20250927084912.2343597-3-ivecera@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>