Extend mlx5dv_get_qp() to return UAR mapping info. This can allow a
process to share its doorbell access with secondary process by re-mmap
the UAR address on the device and make it accessible as a user space
address.

Signed-off-by: Xueming Li <xuemingl@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>

[khng: Cherry-picked from rdma-core
14d0beecc574d80e17131bded11e18f39d45ae46]