Add VF rate limit feature

This patch enhances the NFP driver to supports assignment of
both max_tx_rate and min_tx_rate to VFs

The template of configurations below is all supported.
e.g.
 # ip link set $DEV vf $VF_NUM max_tx_rate $RATE_VALUE
 # ip link set $DEV vf $VF_NUM min_tx_rate $RATE_VALUE
 # ip link set $DEV vf $VF_NUM max_tx_rate $RATE_VALUE \
			       min_tx_rate $RATE_VALUE
 # ip link set $DEV vf $VF_NUM min_tx_rate $RATE_VALUE \
			       max_tx_rate $RATE_VALUE

The max RATE_VALUE is limited to 0xFFFF which is about
63Gbps (using 1024 for 1G)
Signed-off-by: Bin Chen <bin.chen@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Baowen Zheng <baowen.zheng@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>