package/linuxptp: bump version to 3.1

package/linuxptp: bump version to 3.1

From release note:

Version 3.1 - September 2020

This release offers substantial new functionality. Users are encouraged
to upgrade. The most notable changes are as follows.

* Fixes non-functional NMEA mode of the ts2phc program (since 3.0)
* Improves system time synchronization via the PTP_SYS_OFFSET_PRECISE
  and PTP_SYS_OFFSET_EXTENDED ioctls.
* Supports implementing a PTP GM clock by using a GPS radio or other
  PPS time source.
* Provides per-port statistics via the management interface.
* Implements IEEE 1588 v2.1 slave event monitoring.
* Adds Automotive Profile support.y

 utils/test-pkg -p linuxptp
                             br-arm-full [1/6]: OK
                  br-arm-cortex-a9-glibc [2/6]: OK
                   br-arm-cortex-m4-full [3/6]: OK
                          br-x86-64-musl [4/6]: OK
                      br-arm-full-static [5/6]: OK
                            sourcery-arm [6/6]: OK
6 builds, 0 skipped, 0 build failed, 0 legal-info failed

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>