eth: mvpp2: implement new RSS context API

Implement the separate create/modify/delete ops for RSS.

No problems with IDs - even tho RSS tables are per device
the driver already seems to allocate IDs linearly per port.
There's a translation table from per-port context ID
to device context ID.

mvpp2 doesn't have a key for the hash, it defaults to
an empty/previous indir table.

Note that there is no key at all, so we don't have to be
concerned with reporting the wrong one (which is addressed
by a patch later in the series).

Compile-tested only.

Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>