ptp: add helpers to get the phc_index by of_node or dev

ptp: add helpers to get the phc_index by of_node or dev

Some Ethernet controllers do not have an integrated PTP timer function.
Instead, the PTP timer is a separated device and provides PTP hardware
clock to the Ethernet controller to use. Therefore, the Ethernet
controller driver needs to obtain the PTP clock's phc_index in its
ethtool_ops::get_ts_info(). Currently, most drivers implement this in
the following ways.

1. The PTP device driver adds a custom API and exports it to the Ethernet
controller driver.
2. The PTP device driver adds private data to its device structure. So
the private data structure needs to be exposed to the Ethernet controller
driver.

When registering the ptp clock, ptp_clock_register() always saves the
ptp_clock pointer to the private data of ptp_clock::dev. Therefore, as
long as ptp_clock::dev is obtained, the phc_index can be obtained. So
the following generic APIs can be added to the ptp driver to obtain the
phc_index.

1. ptp_clock_index_by_dev(): Obtain the phc_index by the device pointer
of the PTP device.
2.ptp_clock_index_by_of_node(): Obtain the phc_index by the of_node
pointer of the PTP device.

Also, we can add another API like ptp_clock_index_by_fwnode() to get the
phc_index by fwnode of PTP device. However, this API is not used in this
patch set, so it is better to add it when needed.

Suggested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250829050615.1247468-4-wei.fang@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>