Second try at adding PTP support

[HarishgunaS]

Background

The company that I am working at requires PTP leader/follower support on an STM32 H7 series microcontroller. This PR adds PTP support for the H7. It is a revised version of #444.

Description

All the PTP related features that have been added in this PR are completely feature gated, so that use without the PTP feature is completely unaffected.

There are three major parts of this implementation.

1. Modifying the Ethernet driver to provide ingress/egress hardware timestamping capabilities.
2. Adding a PTP driver that assists with timekeeping in the Ethernet PTP peripheral.
3. Adding a PTP frame buffer and accessor methods to the EthernetDMA, since smoltcp would drop any PTP frames on interface.poll.

The author of a related PR on a different repo had concerns about the Rx pipeline being stalled due to timestamps being stored in a context descriptor that is owned by the application and blocking the DMA. To minimize concerns regarding this, I ran a ping flood against the microcontroller using the ptp-take2 branch HAL as well as the main branch HAL, and the results were basically identical.

Validation

Wireshark output of packets emitted by the micro, showing timestamped PTP followup packets.

Ping flooding the micro with no packet loss.

[HarishgunaS]

Hey @richardeoin, this PR is ready. Please review if and when you have time. Thanks!

[jmgrosen]

@richardeoin I'd love if you could review and merge this as well!