common/sfc_efx/base: add function control window concept

common/sfc_efx/base: add function control window concept

Function control window can be located at a different offset than
other windows on Riverhead. Meaning that the drivers must handle
accesses to the function control window differently in case of EF100.

Add accessor macros for function control window and change
EFX NIC create API to facilitate that accessors.

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