nfp: add repr_preclean callback

Just before a repr is cleaned up, we give the app a chance to perform
some preclean configuration while the reprs pointer is still configured
for the app.

Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>