nfp: bpf: add verification and codegen for map lookups

Verify our current constraints on the location of the key are
met and generate the code for calling map lookup on the datapath.

New relocation types have to be added - for helpers and return
addresses.

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