IB/uverbs: Add device memory registration ioctl support

Adding new ioctl method for the MR object - REG_DM_MR.

This command can be used by users to register an allocated
device memory buffer as an MR and receive lkey and rkey
to be used within work requests.

It is added as a new method under the MR object and using a new
ib_device callback - reg_dm_mr.
The command creates a standard ib_mr object which represents the
registered memory.

Signed-off-by: Ariel Levkovich <lariel@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>