mlxsw: reg: Add Policy Engine Multicast Router Binding Table Register

In Spectrum-2, multicast routing is implemented explicitly using policy
engine (ACL) block. PEMRBT register is used to bind a dedicated ACL group
to a specific IP protocol.
Add the register to be later used in multicast router implementation.

Signed-off-by: Nir Dotan <nird@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>