net: ethernet: ti: am65-cpts: Enable RX HW timestamp for PTP packets using CPTS FIFO

Add a new function "am65_cpts_rx_timestamp()" which checks for PTP
packets from header and timestamps them.

Add another function "am65_cpts_find_rx_ts()" which finds CPTS FIFO
Event to get the timestamp of received PTP packet.

Signed-off-by: Chintan Vankar <c-vankar@ti.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>