lightnvm: complete geo structure with maxoc*

Complete the generic geometry structure with the maxoc and maxocpu
felds, present in the 2.0 spec. Also, expose them through sysfs.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>