nfp: bpf: add map updates from the datapath

Support calling map_update_elem() from the datapath programs
by calling into FW-provided helper.  Value pointer is passed
in LM pointer #2.  Keeping track of old state for arg3 is not
necessary, since LM pointer #2 will be always loaded in this
case, the trivial optimization for value at the bottom of the
stack can't be done here.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>