device/Kconfig: Adapt PCIEXP_HOTPLUG_BUSES

The default of 32 buses per hotplug bridge is rather high. Especially
for platforms that limit MMConf space to 64 buses: they run out of
numbers if there is more than a single hotplug bridge.

Lower the default to

*  8 if MMConf is limited to  64 or less buses,
* 16 if MMConf is limited to 128 or less buses.

Change-Id: I06d522dd92ceea9f4798273b26f947a5333800c3
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/52069
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/device/Kconfig

@@ -626,6 +626,8 @@ if PCIEXP_HOTPLUG
 
 config PCIEXP_HOTPLUG_BUSES
 	int "PCI Express Hotplug Buses"
+	default  8 if MMCONF_SUPPORT && MMCONF_BUS_NUMBER <=  64
+	default 16 if MMCONF_SUPPORT && MMCONF_BUS_NUMBER <= 128
 	default 32
 	help
 	  This is the number of buses allocated for hotplug PCI express