nfp: bpf: implement byte swap instruction

Implement byte swaps with rotations, shifts and byte loads.
Remember to clear upper parts of the 64 bit registers.

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>