net/mlx5: Add reserved-gids support

Reserved GIDs are entries in the GID table in use by the mlx5_core
and its submodules (e.g. FPGA, SRIOV, E-Swtich, netdev).
The entries are reserved at the high indexes of the GID table.

A mlx5 submodule may reserve a certain amount of GIDs for its own use
during the load sequence by calling mlx5_core_reserve_gids, and must
also take care to un-reserve these GIDs when it closes.
Reservation is only allowed during the load sequence and before any
interfaces (e.g. mlx5_ib or mlx5_en) are up.

After reservation, a submodule may call mlx5_core_reserved_gid_alloc/
free to allocate entries from the reserved GIDs pool.

Reserve a GID table entry for every supported FPGA QP.

A later patch in the patchset will remove them from being reported to
IB core.
Another such patch will make use of these for FPGA QPs in Innova NIC.

Added lib/mlx5.h to serve as a library for mlx5 submodlues, and to
expose only public mlx5 API, more mlx5 library files will be added in
future submissions.

Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>