nfp: bpf: encode extended LM pointer operands

Most instructions have special fields which allow switching
between base and extended Local Memory pointers.  Introduce
those to register encoding, we will use the extra LM pointers
to access high addresses of the stack.

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>