common/sfc_efx/base: add function control window lookup API

common/sfc_efx/base: add function control window lookup API

Riverhead NIC may provide a locator of function control window
(EF100 resource locator).
The locator may be present in a Xilinx capabilities table which
itself is located by looking into extended PCI capabilities.

PCI capabilities are made possible to access by adding PCI config
space interface to efsys.

APIs are implemented to facilitate function control window lookup:
 - API to find an extended PCI capability given a capability ID;
 - API to read Xilinx PCI capability;

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>