net/mlx5: Bridge, expose FDB state via debugfs

For debugging purposes expose offloaded FDB state (flags, counters, etc.)
via debugfs inside 'esw' root directory. Example debugfs file output:

$ cat mlx5/0000\:08\:00.0/esw/bridge/bridge1/fdb
DEV              MAC               VLAN              PACKETS                BYTES              LASTUSE FLAGS
enp8s0f0_1       e4:0a:05:08:00:06    2                    2                  204           4295567112   0x0
enp8s0f0_0       e4:0a:05:08:00:03    2                    3                  278           4295567112   0x0

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>