nfp: bpf: support arithmetic right shift by constant (BPF_ARSH | BPF_K)

Code logic is similar with logic right shift except we also need to set
PREV_ALU result properly, the MSB of which is the bit that will be
replicated to fill in all the vacant positions.

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