nfp: bpf: add stack write support

Stack is implemented by the LMEM register file.  Unaligned accesses
to LMEM are not allowed.  Accesses also have to be 4B wide.

To support stack we need to make sure offsets of pointers are known
at translation time (for now) and perform correct load/mask/shift
operations.

Since we can access first 64B of LMEM without much effort support
only stacks not bigger than 64B.  Following commits will extend
the possible sizes beyond that.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>