Introduce packet pacing uobject and its alloc and destroy
methods.

This uobject holds mlx5 packet pacing context according to the device
specification and enables managing packet pacing device entries that are
needed by DEVX applications.

Link: https://lore.kernel.org/r/20200219190518.200912-3-leon@kernel.orgSigned-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>