sfc: skeleton EF100 PF driver

No TX or RX path, no MCDI, not even an ifup/down handler.
Besides stubs, the bulk of the patch deals with reading the Xilinx
extended PCIe capability, which tells us where to find our BAR.

Though in the same module, EF100 has its own struct pci_driver,
which is named sfc_ef100.

A small number of additional nic_type methods are added; those in the
TX (tx_enqueue) and RX (rx_packet) paths are called through indirect
call wrappers to minimise the performance impact.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>