net: devlink: report cell size of shared buffers

Shared buffer allocation is usually done in cell increments.
Drivers will either round up the allocation or refuse the
configuration if it's not an exact multiple of cell size.
Drivers know exactly the cell size of shared buffer, so help
out users by providing this information in dumps.

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