nfp: bpf: encode LMEM accesses

NFP LMEM is a large, indirectly accessed register file.  There
are two basic indirect access registers.  Each access operation
may either use offset (up to 8 or 16 words) or perform post
decrement/increment.

Add encodings of LMEM indexes as instruction operands.

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>