RDMA/mlx5: Add support to MODIFY_MEMIC command

Add two functions to allocate and deallocate MEMIC operations
by using the MODIFY_MEMIC command.

Link: https://lore.kernel.org/r/20210411122924.60230-6-leon@kernel.org
Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>