net/mlx5: Add interface to get reference to a UAR

net/mlx5: Add interface to get reference to a UAR

A reference to a UAR is required to generate CQ or EQ doorbells. Since
CQ or EQ doorbells can all be generated using the same UAR area without
any effect on performance, we are just getting a reference to any
available UAR, If one is not available we allocate it but we don't waste
the blue flame registers it can provide and we will use them for
subsequent allocations.
We get a reference to such UAR and put in mlx5_priv so any kernel
consumer can make use of it.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>