docs: ptp.txt: convert to ReST and move to driver-api

The conversion is trivial: just adjust title markups.

In order to avoid conflicts, let's add an :orphan: tag
to it, to be removed when this file gets added to the
driver-api book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>