usb: cdc-acm: add PPS support

This patch adds support for PPS to CDC devices. Changes to the DCD pin
are monitored and passed to the ldisc system, which is used by
pps-ldisc.

Signed-off-by: Dan Drown <dan-netdev@drown.org>
Link: https://lore.kernel.org/r/ZM8ExV6bAvJtIA1d@vps3.drown.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>