cxgbe: update doc for FreeBSD support

cxgbe: update doc for FreeBSD support

Update cxgbe documentation to include support for FreeBSD:
1. Add instructions on how to compile CXGBE PMD in FreeBSD.
2. Add instructions on how to flash firmware image on Chelsio T5 cards in
   FreeBSD.
3. Add sample application usage for FreeBSD.
4. Add an extra step to reload kernel module in Linux in order for the new
   firmware to be loaded.
5. Typo fixes.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Kumar Sanghvi <kumaras@chelsio.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>