sfc: Add support for SFC9000 family (2)

This integrates support for the SFC9000 family of 10G Ethernet
controllers and LAN-on-motherboard chips, starting with the SFL9021
'Siena' and SFC9020 'Bethpage'.

Credit for this code is largely due to my colleagues at Solarflare:

   Guido Barzini
   Steve Hodgson
   Kieran Mansley
   Matthew Slattery
   Neil Turton

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>