dt-bindings: ptp: add NETC Timer PTP clock

dt-bindings: ptp: add NETC Timer PTP clock

NXP NETC (Ethernet Controller) is a multi-function PCIe Root Complex
Integrated Endpoint (RCiEP), the Timer is one of its functions which
provides current time with nanosecond resolution, precise periodic
pulse, pulse on timeout (alarm), and time capture on external pulse
support. And also supports time synchronization as required for IEEE
1588 and IEEE 802.1AS-2020. So add device tree binding doc for the PTP
clock based on NETC Timer.

NETC Timer has three reference clock sources, but the clock mux is inside
the IP. Therefore, the driver will parse the clock name to select the
desired clock source. If the clocks property is not present, NETC Timer
will use the system clock of NETC IP as its reference clock. Because the
Timer is a PCIe function of NETC IP, the system clock of NETC is always
available to the Timer.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20250829050615.1247468-2-wei.fang@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>