net/mlx5: Introduce blue flame register allocator

Here is an implementation of an allocator that allocates blue flame
registers. A blue flame register is used for generating send doorbells.
A blue flame register can be used to generate either a regular doorbell
or a blue flame doorbell where the data to be sent is written to the
device's I/O memory hence saving the need to read the data from memory.
For blue flame kind of doorbells to succeed, the blue flame register
need to be mapped as write combining. The user can specify what kind of
send doorbells she wishes to use. If she requested write combining
mapping but that failed, the allocator will fall back to non write
combining mapping and will indicate that to the user.
Subsequent patches in this series will make use of this allocator.

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>