dpll: zl3073x: add ref-sync pair support

dpll: zl3073x: add ref-sync pair support

Add support for ref-sync pair registration using the 'ref-sync-sources'
phandle property from device tree. A ref-sync pair consists of a clock
reference and a low-frequency sync signal where the DPLL locks to the
clock reference but phase-aligns to the sync reference.

The implementation:
- Stores fwnode handle in zl3073x_dpll_pin during pin registration
- Adds ref_sync_get/set callbacks to read and write the sync control
  mode and pair registers
- Validates ref-sync frequency constraints: sync signal must be 8 kHz
  or less, clock reference must be 1 kHz or more and higher than sync
- Excludes sync source from automatic reference selection by setting
  its priority to NONE on connect; on disconnect the priority is left
  as NONE and the user must explicitly make the pin selectable again
- Iterates ref-sync-sources phandles to register declared pairings
  via dpll_pin_ref_sync_pair_add()

Reviewed-by: Petr Oros <poros@redhat.com>
Reviewed-by: Prathosh Satish <Prathosh.Satish@microchip.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20260408102716.443099-6-ivecera@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>