ethdev: add flow API to expand RSS flows

Introduce an helper for PMD to expand easily flows items list with RSS
action into multiple flow items lists with priority information.

For instance a user items list being "eth / end" with rss action types
"ipv4-udp ipv6-udp end" needs to be expanded into three items lists:

 - eth
 - eth / ipv4 / udp
 - eth / ipv6 / udp

to match the user request.  Some drivers are unable to reach such
request without this expansion, this API is there to help those.
Only PMD should use such API for their internal cooking, the application
will still handle a single flow.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>