net: mvpp2: ptp: add support for receive timestamping

net: mvpp2: ptp: add support for receive timestamping

Add support for receive timestamping. When enabled, the hardware adds
a timestamp into the receive queue descriptor for all received packets
with no filtering. Hence, we can only support NONE or ALL receive
filter modes.

The timestamp in the receive queue contains two bit sof seconds and
the full nanosecond timestamp. This has to be merged with the remainder
of the seconds from the TAI clock to arrive at a full timestamp before
we can convert it to a ktime for the skb hardware timestamp field.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>