doc: add FAQ instructions on specific firmware install

FAQ instructions used to inform user how to install the latest firmware
version for their NIC. End user may want to install a particular
version, so add steps detailing how to do so.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Yefeng Lin <yefeng.lin@arm.com>
Change-Id: I6d1fca72bfea602d6d418e1f140b482fd3baa80e