ethdev: support sub-function representor

ethdev: support sub-function representor

SubFunction is a portion of the PCI device, created on demand, a SF
netdev has its own dedicated queues(txq, rxq). A SF netdev supports
eswitch representation offload similar to existing PF and VF
representors.

To support SF representor, this patch introduces new devargs syntax,
examples:
 representor=sf0               - single SubFunction representor
 representor=sf[1,3,5]         - single list
 representor=sf[0-3],          - single range
 representor=sf[0,2-6,8,10-12] - list with singles and ranges

Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>