rocker: move OF-DPA stuff into separate file

Carve out OF-DPA would specific code from the common file to the world
file. This change required struct rocker and struct rocker_port split
into world specific struct ofdpa and struct ofdpa_port. Along with this
the world specific functions and defines were renamed from prefix
"rocker_" to "ofdpa_".

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>