mlxsw: reg: Add the Switch Filtering DB Flush register

When removing a net device from a bridge we should flush the FDB entries
associated with this net device. Up until now, we relied upon bridge
code to do that for us, but it is possible for user to prevent hardware
from syncing with the software bridge (learning_sync=0), so we need to
flush overselves.

Add the Switch Filtering DB Flush (SFDF) register that is used to flush
FDB entries according to different parameters (per-port, per-FID etc).

Fixes: 56ade8fe

 ("mlxsw: spectrum: Add initial support for Spectrum ASIC")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>