nfp: bpf: add map data structure

To be able to split code into reasonable chunks we need to add
the map data structures already.  Later patches will add code
piece by piece.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>