net/mlx5: Expose MPEGC (Management PCIe General Configuration) structures

This patch exposes PRM layout for handling MPEGC (Management PCIe
General Configuration).

This will be used in the downstream patch for configuring MPEGC via the
driver.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>