mlxsw: reg: Add Switch FID Management register definition

Filtering identifiers (FIDs) are unique identifers of bridge instances
in the hardware.

Add the SFMR register, which is responsible for the creation and
configuration of these FIDs.

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>