nfp: add RTsym access helpers

RTsyms may have special encodings for more complex symbol types.
For example symbols which are placed in external memory unit's
cache directly, constants or local memory.  Add set of helpers
which will check for those special encodings and handle them
correctly.

For now only add direct cache accesses, we don't have a need to
access the other ones in foreseeable future.

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>