sfc: MAC TX timestamp handling on the 8000 series

sfc: MAC TX timestamp handling on the 8000 series

BugLink: https://bugs.launchpad.net/bugs/1836635
TX timestamps on 8000 series are supplied from the MAC. This timestamp is
 only 48 bits long. The high order bits from the last time sync event are
 used for the top 16 bits.

Signed-off-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit c1d0d33946725775be1c68515c07d0ff8237d222)
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>