cxgbe(4): New knob to limit driver to the specified types of doorbells.

hw.cxgbe.doorbells_allowed="0xf"

The adapter's doorbells bitmap is clipped to the value specified in the
tunable, which is meant for debug and workarounds only. There is no
change in default behavior.

MFC after: 1 week
Sponsored by: Chelsio Communications