sfc: Prepare to bind the sfc driver to the VF.

Added efx_nic_type structure for VF.
Mapped a different BAR for VF as it uses BAR 0 for memory.
Added functions sriov_init and sriov_fini.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>