Documentation: nfp: add nfp driver specific notes

This adds the initial documentation for the NFP driver specific
documentation.

Right now, only basic information is provided about acquiring firmware
and configuring device firmware loading.

Original driver documentation can be found here:
https://github.com/Netronome/nfp-drv-kmods/blob/master/README.md

Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>