dpll: zl3073x: Add output pin frequency helper

dpll: zl3073x: Add output pin frequency helper

Introduce zl3073x_dev_output_pin_freq_get() helper function to compute
the output pin frequency based on synthesizer frequency, output divisor,
and signal format. For N-div signal formats, the N-pin frequency is
additionally divided by esync_n_period.

Add zl3073x_out_is_ndiv() helper to check if an output is configured
in N-div mode (2_NDIV or 2_NDIV_INV signal formats).

Refactor zl3073x_dpll_output_pin_frequency_get() callback to use the
new helper, reducing code duplication and enabling reuse of the
frequency calculation logic in other contexts.

This is a preparatory change for adding current frequency to the
supported frequencies list in pin properties.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20260205154350.3180465-2-ivecera@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>