net/mlx5: Bridge, add offload infrastructure

Create new files bridge.{c|h} in en/rep directory that implement bridge
interaction with representor netdevices and handle required
events/notifications, bridge.{c|h} in esw directory that implement all
necessary eswitch offloading infrastructure and works on vport/eswitch
level. Provide new kconfig MLX5_BRIDGE which is automatically selected when
both kernel bridge and mlx5 eswitch configs are enabled.

Provide basic infrastructure for bridge offloads:

- struct mlx5_esw_bridge_offloads - per-eswitch bridge offload structure
that encapsulates generic bridge-offloads data (notifier blocks, ingress
flow table/group, etc.) that is created/deleted on enable/disable eswitch
offloads.

- struct mlx5_esw_bridge - per-bridge structure that encapsulates
per-bridge data (reference counter, FDB, egress flow table/group, etc.)
that is created when first eswitch represetor is attached to new bridge and
deleted when last representor is removed from the bridge as a result of
NETDEV_CHANGEUPPER event.

The bridge tables are created with new priority FDB_BR_OFFLOAD in FDB
namespace. The new priority is between tc-miss and slow path priorities.
Priority consist of two levels: the ingress table that is global per
eswitch and matches incoming packets by src_mac/vid and redirects them to
next level (egress table) that is chosen according to ingress port bridge
membership and matches on dst_mac/vid in order to redirect packet to vport
according to the following diagram:

                +
                |
      +---------v----------+
      |                    |
      |   FDB_TC_OFFLOAD   |
      |                    |
      +---------+----------+
                |
                |
      +---------v----------+
      |                    |
      |   FDB_FT_OFFLOAD   |
      |                    |
      +---------+----------+
                |
                |
      +---------v----------+
      |                    |
      |    FDB_TC_MISS     |
      |                    |
      +---------+----------+
                |
+--------------------------------------+
|               |                      |
|        +------+                      |
|        |                             |
| +------v--------+   FDB_BR_OFFLOAD   |
| | INGRESS_TABLE |                    |
| +------+---+----+                    |
|        |   |      match              |
|        |   +---------+               |
|        |             |               |    +-------+
|        |     +-------v-------+ match |    |       |
|        |     | EGRESS_TABLE  +------------> vport |
|        |     +-------+-------+       |    |       |
|        |             |               |    +-------+
|        |    miss     |               |
|        +------+------+               |
|               |                      |
+--------------------------------------+
                |
                |
      +---------v----------+
      |                    |
      |   FDB_SLOW_PATH    |
      |                    |
      +---------+----------+
                |
                v

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Jianbo Liu <jianbol@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>