docs: update details for Mellanox ConnectX-5

OFED driver has to be installed and then NIC firmware is required to be
updated if running solution in a fresh installed environment.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: If1544549e6f24382fc9f17d27750c1a2d450215e