pci: Make HW.PCI.MMConf a public generic

GCC 8 wants us to tie the hidden, private state of HW.PCI.MMConf to a
surrounding state. Though, as it's impossible to do that before the
package instantiation, we can only comply by making it public.

Change-Id: I41c3d1324a7d9acde5be2c246011f976361e8f2b
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/26304
Tested-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Author: i-c-o-n

common/hw-pci-mmconf.ads

@@ -17,7 +17,7 @@ with HW.MMIO_Range;
 
 pragma Elaborate_All (HW.MMIO_Range);
 
-private generic
+generic
    Dev : Address := (0, 0, 0);
 package HW.PCI.MMConf
 with