cxgb4: Use BAR2 Going To Sleep (GTS) for T5 and later.

Use BAR2 GTS for T5. If we are on T4 use the old doorbell mechanism;
otherwise ue the new BAR2 mechanism. Use BAR2 doorbells for refilling FL's.

Based on original work by Casey Leedom <leedom@chelsio.com>

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>