net: ti: prueth: Add support for reading timestamps from rx packets

When PTP packets are received on ICSS ports, dual_emac firmware record the
timestamps and appends the timestamp at the next 32 byte block after the end
of packet. Add support for reading this timestamp from packet and pass
it to userspace layer.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>