common/sfc_efx/base: add Xilinx capabilities table lookup

common/sfc_efx/base: add Xilinx capabilities table lookup

APIs for searching a capability in a Xilinx capabilities table and
reading table entries are needed for function control window lookup
to get the bar index and offset of the window.

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