nfp: move the fast path code to separate files

In preparation for support for a new datapath format move all
ring and fast path logic into separate files. It is basically
a verbatim move with some wrapping functions, no new structures
and functions added.

The current data path is called NFD3 from the initial version
of the driver ABI it used. The non-fast path, but ring related
functions are moved to nfp_net_dp.c file.

Changes to Jakub's work:
* Rebase on xsk related code.
* Split the patch, move the callback changes to next commit.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>