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 <[e-mail]>
Acked-by: Richard Cochran <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/networking/timestamping.txt

···
 Instead, expose the hardware clock device on the NIC directly as
 a HW PTP clock source, to allow time conversion in userspace and
 optionally synchronize system time with a userspace PTP stack such
-as linuxptp. For the PTP clock API, see Documentation/ptp/ptp.txt.
+as linuxptp. For the PTP clock API, see Documentation/driver-api/ptp.rst.
 
 Note that if the SO_TIMESTAMP or SO_TIMESTAMPNS option is enabled
 together with SO_TIMESTAMPING using SOF_TIMESTAMPING_SOFTWARE, a false

Documentation/ptp/ptp.txt Documentation/driver-api/ptp.rst

···
+:orphan:
 
-* PTP hardware clock infrastructure for Linux
+===========================================
+PTP hardware clock infrastructure for Linux
+===========================================
 
   This patch set introduces support for IEEE 1588 PTP clocks in
   Linux. Together with the SO_TIMESTAMPING socket options, this
···
     - Period output signals configurable from user space
     - Synchronization of the Linux system time via the PPS subsystem
 
-** PTP hardware clock kernel API
+PTP hardware clock kernel API
+=============================
 
    A PTP clock driver registers itself with the class driver. The
    class driver handles all of the dealings with user space. The
···
    development, it can be useful to have more than one clock in a
    single system, in order to allow performance comparisons.
 
-** PTP hardware clock user space API
+PTP hardware clock user space API
+=================================
 
    The class driver also creates a character device for each
    registered clock. User space can use an open file descriptor from
···
    ancillary clock features. User space can receive time stamped
    events via blocking read() and poll().
 
-** Writing clock drivers
+Writing clock drivers
+=====================
 
    Clock drivers include include/linux/ptp_clock_kernel.h and register
    themselves by presenting a 'struct ptp_clock_info' to the
···
    class driver, since the lock may also be needed by the clock
    driver's interrupt service routine.
 
-** Supported hardware
+Supported hardware
+==================
 
-   + Freescale eTSEC gianfar
+   * Freescale eTSEC gianfar
+
      - 2 Time stamp external triggers, programmable polarity (opt. interrupt)
      - 2 Alarm registers (optional interrupt)
      - 3 Periodic signals (optional interrupt)
 
-   + National DP83640
+   * National DP83640
+
      - 6 GPIOs programmable as inputs or outputs
      - 6 GPIOs with dedicated functions (LED/JTAG/clock) can also be
        used as general inputs or outputs
···
      - GPIO outputs can produce periodic signals
      - 1 interrupt pin
 
-   + Intel IXP465
+   * Intel IXP465
+
      - Auxiliary Slave/Master Mode Snapshot (optional interrupt)
      - Target Time (optional interrupt)

MAINTAINERS

···
 S:	Maintained
 W:	http://linuxptp.sourceforge.net/
 F:	Documentation/ABI/testing/sysfs-ptp
-F:	Documentation/ptp/*
+F:	Documentation/driver-api/ptp.rst
 F:	drivers/net/phy/dp83640*
 F:	drivers/ptp/*
 F:	include/linux/ptp_cl*