nfp: save the MU locality field offset

We will soon need the MU locality field offset much more
often than just for decoding MIP address.  Save it in nfp_cpp
for quick access.  Note that we can already reuse the target
config from nfp_cpp, no need to do the XPB read.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Francois H. Theron <francois.theron@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>