mlxsw: spectrum_switchdev: Add support for maintaining hash table of MDB entries

Currently MDB entries are stored in a list as part of
'struct mlxsw_sp_bridge_device'. Storing them in a hash table in
addition to the list will allow finding a specific entry more efficiently.

Add support for the required hash table, the next patches will insert
and remove MDB entries from the table. The existing code which adds and
removes entries will be removed and replaced by new code in the next
patches, so there is no point to adjust the existing code.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>