platform/x86/intel/vsec: Add base address field

Some devices may emulate PCI VSEC capabilities in MMIO. In such cases the
BAR is not readable from a config space. Provide a field for drivers to
indicate the base address to be used.

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20231129222132.2331261-9-david.e.box@linux.intel.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>