mlxsw: spectrum_buffers: Add ability to veto TC's configuration

Subsequent patches are going to need to veto changes in certain TCs'
binding and threshold configurations.

Add fields to the TC's struct that indicate if the TC can be bound to a
different pool and whether its threshold can change and enforce that.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>