dpll: add frequency monitoring to netlink spec

dpll: add frequency monitoring to netlink spec

Add DPLL_A_FREQUENCY_MONITOR device attribute to allow control over
the frequency monitor feature. The attribute uses the existing
dpll_feature_state enum (enable/disable) and is present in both
device-get reply and device-set request.

Add DPLL_A_PIN_MEASURED_FREQUENCY pin attribute to expose the measured
input frequency in millihertz (mHz). The attribute is present in the
pin-get reply. Add DPLL_PIN_MEASURED_FREQUENCY_DIVIDER constant to
allow userspace to extract integer and fractional parts.

Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20260402184057.1890514-2-ivecera@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>