nfp: move area mapping helper into nfpcore

nfp_net_map_area() is a helper for mapping areas of NFP memory
defined in nfp_net_main.c.  Move it to nfpcore to allow reuse
and rename accordingly.  Create an additional helper -
nfp_cpp_area_alloc_acquire() the opposite of already existing
nfp_cpp_area_release_free().

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>