dpll: add fractional frequency offset to pin-parent-device

Add both fractional-frequency-offset (PPM) and
fractional-frequency-offset-ppt (PPT) attributes to the
pin-parent-device nested attribute set, alongside the existing
top-level pin attributes. Both carry the same measurement at
different precisions.

Introduce enum dpll_ffo_type and struct dpll_ffo_param to
distinguish FFO contexts: DPLL_FFO_PORT_RXTX_RATE for the RX vs
TX symbol rate offset reported at the top level, and
DPLL_FFO_PIN_DEVICE for the pin vs parent DPLL offset reported
in the pin-parent-device nest.

Add a supported_ffo bitmask to struct dpll_pin_ops so drivers
declare which FFO types they support. The core only calls ffo_get
for types the driver has opted into, eliminating the need for
per-driver NULL pointer guards. Validate at pin registration time
that supported_ffo is not set without an ffo_get callback.

Update mlx5 (DPLL_FFO_PORT_RXTX_RATE) and zl3073x
(DPLL_FFO_PORT_RXTX_RATE) drivers to use the new API.

Add documentation for both FFO types to dpll.rst.

Changes v3 -> v4:
- Replace dpll=NULL overloading with enum dpll_ffo_type and
  struct dpll_ffo_param (Jakub Kicinski)
- Add supported_ffo opt-in bitmask in dpll_pin_ops for fail-close
  driver validation (Jakub Kicinski)
- Add WARN_ON in dpll_pin_register for supported_ffo without
  ffo_get callback

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20260511155816.99936-2-ivecera@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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