nfp: bpf: move to datapath ABI version 2

Datapath ABI version 2 stores the packet information in LMEM
instead of NNRs.  We also have strict restrictions on which
GPRs we can use.  Only GPRs 0-23 are reserved for BPF.

Adjust the static register locations and "ABI" registers.
Note that packet length is packed with other info so we have
to extract it into one of the scratch registers, OTOH since
LMEM can be used in restricted operands we don't have to
extract packet pointer.

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>