nfp: add helper for mapping runtime symbols

Move most of the helper for mapping RTsyms from nfp_net_main.c
to nfpcore.  Use the new helper directly for mapping MAC statistics,
since they don't need to include the PCIe interface ID in the symbol
name.

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>