nfp: separate high level and low level NSP headers

We will soon add more NSP commands and structure definitions.
Move all high-level NSP header contents to a common nfp_nsp.h file.
Right now it mostly boils down to renaming nfp_nsp_eth.h and
moving some functions from nfp.h there.

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