ptp: add debugfs interfaces to loop back the periodic output signal

ptp: add debugfs interfaces to loop back the periodic output signal

For some PTP devices, they have the capability to loop back the periodic
output signal for debugging, such as the ptp_qoriq device. So add the
generic interfaces to set the periodic output signal loopback, rather
than each vendor having a different implementation.

Show how many channels support the periodic output signal loopback:
$ cat /sys/kernel/debug/ptp<N>/n_perout_loopback

Enable the loopback of the periodic output signal of channel X:
$ echo <X> 1 > /sys/kernel/debug/ptp<N>/perout_loopback

Disable the loopback of the periodic output signal of channel X:
$ echo <X> 0 > /sys/kernel/debug/ptp<N>/perout_loopback

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Link: https://patch.msgid.link/20250905030711.1509648-2-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>