lightnvm: enable to send hint to erase command

Erases might be subject to host hints. An example is multi-plane
programming to erase blocks in parallel. Enable targets to specify this
hint.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>