net: atlantic: add support for 64-bit reads/writes

This patch adds support for 64-bit reads/writes where applicable, e.g.
A2 supports them.

Signed-off-by: Pavel Belous <pbelous@marvell.com>
Signed-off-by: Mark Starovoytov <mstarovoitov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>