Add OXM data to mf_fields

Add OXM data to mf_fields

Add oxm_name and oxm_header elements to struct mf_field
and populate those entries for fields that are present
in both NXM and the OXM basic class.

This implementation was suggested by Ben Pfaff.

This does not address any possible differences in the NXM and
OXM basic class fields, for instance different maskability.
That may be addressed later as needed.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>