pci: brcmstb: Support different variants using a cfg struct

pci: brcmstb: Support different variants using a cfg struct

The Linux kernel driver already had support for multiple hardware
variants when the bcm2712 was added (see e.g. linux commit
10dbedad3c818 which is the last in a longer set of changes). This
patch brings in this required infrastructure and adds a
differentiation between 2711 and 2712 register layouts on top.

Signed-off-by: Torsten Duwe <duwe@suse.de>
Co-authored-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com>
Tested-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>