dpll: zl3073x: implement frequency monitoring

dpll: zl3073x: implement frequency monitoring

Extract common measurement latch logic from zl3073x_ref_ffo_update()
into a new zl3073x_ref_freq_meas_latch() helper and add
zl3073x_ref_freq_meas_update() that uses it to latch and read absolute
input reference frequencies in Hz.

Add meas_freq field to struct zl3073x_ref and the corresponding
zl3073x_ref_meas_freq_get() accessor. The measured frequencies are
updated periodically alongside the existing FFO measurements.

Add freq_monitor boolean to struct zl3073x_dpll and implement the
freq_monitor_set/get device callbacks to enable/disable frequency
monitoring via the DPLL netlink interface.

Implement measured_freq_get pin callback for input pins that returns the
measured input frequency in mHz.

Reviewed-by: Petr Oros <poros@redhat.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20260402184057.1890514-4-ivecera@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>