dpll: add phase-offset-avg-factor device attribute to netlink spec

Add dpll device level attribute DPLL_A_PHASE_OFFSET_AVG_FACTOR to allow
control over a calculation of reported phase offset value. Attribute is
present, if the driver provides such capability, otherwise attribute
shall not be present.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20250927084912.2343597-2-ivecera@redhat.com


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