docs: vpp: use dpdk driver instead of rdma driver

Use Mellanox DPDK driver in VPP quickstart and user guide due to
stabilty issue in native rdma driver.

Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: Id2df226965244941216ceb33ae2c4027904d8bc7